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Thread: Black (I think) Algae that will not go away

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    drewh's Avatar
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    Black (I think?) Algae that will not go away - any help appreciated!

    Hi all,

    Great resource here helping each other - have often read it but only posting now.

    I have a major problem I'd be most grateful for any help with. This pool was one my late father built (structural engineer), so it has extra special meaning to me. Its a 9,200 gal / 35,000 L, SWG, concrete/plaster (marblesheen is the term here in Australia), sand filter.

    From last year/season and continuing now, I've had an endless problem with what looks like black algae (but its not limited to just small spots, and keeps reappearing in these patches that easily brush away but reappear within 1-2 days and get darker). I've been to my local pool shop and spent hundreds of dollars on their 'recommendations' - everything from copper based algaecide, multiple phosphate starvers, pool clarifier, tried bombing it with liquid chlorine to a high level (15+) also tried cal hypo bombing, kept pH lower around 7.0 to help fight the algae also. The copper algaecide helped only SLIGHTLY, but still, almost every day or two I'd be going and brushing the algae away but it just continued to reappear. Even with the chlorine very high, it still reappears and seems resistant to it. During last season I've tested the water weekly and everything has been within proper levels (CYA about 60, chlorine always at least 6-8, alkalinity always around 100 +/- 5-10.). Currently there is no copper algaecide left in the pool just for reference, CYA is about 40. Pics are taken all recently (last few weeks).

    A few observations I've noticed:
    - the level of calcium (CH) in the pool has repeatedly been 350+ in every test. I know concrete/marblesheen pools need higher calcium so haven't worried about it. The SWG often gets calcium deposits on the plates (less when I turned it slightly down over winter). In just a few spots in the pool, there is like calcium buildup (a slightly mound) and sometimes the black algae/patches will appear near these. Probably only 2-3 of these lumps though.
    - there is two small spots in the deepend that look like actual black spot algae. I've bought triclor pucks and will brush them using a wire brush while leaving a puck on each spot for 30mins-1 hour this weekend. Nowhere else are there actual tight small black spots.
    - on the outer edge of the pool, the black algae patches quite often appear in areas where the pool surface looks rougher. However, also is areas perfectly smooth it also appears sometimes darker (see pics).
    - the shallow end pool steps have a metal handrail, and one of the bolts is slightly rusting (small patch of rust around it thats underwater). I mention this in case it has some relevance.

    Please see the pics (ignore the few leaves near the pressure relief below to help identify it. I'd be most grateful for any help or guidance here - if anyone can work out what it is, its you guys vs the pool shop, clearly they have not been able to solve the problem at all. Thanks so much, Andrew.





    IG 9200 gal / 35000L Marblesheen / Plaster Pool, 24" Hayward Pro Series S244T2 Sand Filter (235LPM), Webster/Speck 1HP Pump, Crystal Clear SWG 7V DC / 30 amp.
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    Re: Black (I think?) Algae that will not go away - any help appreciated!

    It does not sound like black algae, if you can brush it away......

    Some/most of it looks organic to me.

    Have you tested for copper?

    How does the pool look now? Do you have/can you take a pic of it just after vacc'ing?

    Please post a current set of test results:

    FC
    CC
    pH
    TA
    CH
    CYA

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    Re: Black (I think?) Algae that will not go away - any help appreciated!

    Hi butterfly,

    Thanks heaps! it's a great community, have read posts on it for some time.

    I will obtain new test results tomorrow and post them here. Yes I agree re the algae, apart from those 2 actual black spots it's just these patches but they're definitely black in colour yet brush away completely before reappearing within a day or two. Will take a pic before/after vacuuming a spot so you can see.
    IG 9200 gal / 35000L Marblesheen / Plaster Pool, 24" Hayward Pro Series S244T2 Sand Filter (235LPM), Webster/Speck 1HP Pump, Crystal Clear SWG 7V DC / 30 amp.
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    Lives down under, in Brisbane, QLD, Australia!

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    Re: Black (I think?) Algae that will not go away - any help appreciated!

    Quote Originally Posted by Butterfly View Post
    Have you tested for copper?
    How does the pool look now? Do you have/can you take a pic of it just after vacc'ing?
    Please post a current set of test results:
    Just went this morning and had the pool tested (very reputable pool shop so test results accurate!):

    FC - 1.75
    CC - Not tested, doesn't seem to be here in Aus (but he said the FC result is net of chloramines etc)
    Copper - 0 (I did have a copper algaecide in last season, but it honestly made little difference, if at all only slightly helping but the algae still appears every few days in the same spot)
    Iron - 0 (there is a slightly rusted bolt on the hand rail in the pool so I wanted to test to make sure no issues there).
    pH - 8.1 (going to add about 1L of muriatic acid tomorrow morning, but in the past I've kept the pH down at 7.0 to help fight the algae but it has made no tangible difference)
    TA - 160 (will reduce with the acid for the pH, last season it was almost always on TA of 100 and still the algae would persistently stay there)
    CH - 238 (this is probably the lowest level in any test - normally over 400/450 ..... incidentally there is probably the least amount of algae spread around the pool, but still alot - coincidence?)
    CYA - 40
    Phosphate - 1.75 (marked as good).

    Re how it looks, see the pics below with explanations -

    1) This shows the algae towards the middle of the pool. That darker smaller stuff as well as the lighter, wider patches. Both just brush away fairly cleanly, and also vacuum up no problems. Usually this smaller black algae is concentrated towards the middle and deep end, the wide type (see bottom left of pic) is all over the pool.
    2) Example of the lighter, wider spots. These are usually a bit darker than the pic looks, but not as heavy dense dark as the stuff at the top.
    3) Another close up of the patches - this one is towards the outside of the pool, and seems to be on an area where the plaster is a little rough (a few spots are like that).
    4 and 5) These are the only two actual black spot algae in the pool - they won't go away or change by brushing the area. Notice the area of darker patch around them? thats very similar if not the same as the other wider patches mentioned. Could these two spots be somehow distributing the other algae that brushes away around the pool?
    6) Another example of the dark patch of algae that bruses away cleanly.
    7) How the section in 1) looks after vacuuming - i.e. algae completely gone with pretty much nothing left behind.

    Sorry this is so long. Really appreciate everyones help. I'm just lost as to what to do having tried almost everything.







    IG 9200 gal / 35000L Marblesheen / Plaster Pool, 24" Hayward Pro Series S244T2 Sand Filter (235LPM), Webster/Speck 1HP Pump, Crystal Clear SWG 7V DC / 30 amp.
    TF-100 Test Kit

    Lives down under, in Brisbane, QLD, Australia!

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    Re: Black (I think) Algae that will not go away

    Just forgot to mention the FC is a bit low because Salt was low after we had alot of rain here - I'm going to add two salt bags so this figure should rise. But even when FC has been 8 and even up in the 15-16 for a week, it made no difference to the algae, it still appeared almost daily in the same spots after being brushed or vacuumed away.
    IG 9200 gal / 35000L Marblesheen / Plaster Pool, 24" Hayward Pro Series S244T2 Sand Filter (235LPM), Webster/Speck 1HP Pump, Crystal Clear SWG 7V DC / 30 amp.
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    Lives down under, in Brisbane, QLD, Australia!

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    Re: Black (I think) Algae that will not go away

    You have a basic misunderstanding of chlorine. It kills algae....period. If what you have there is algae, chlorine will kill it.

    My suggestion would be to read the ABC's of pool water chemistry in Pool School to get a better understanding of how your pool is reacting.

    Then, clean your pool thoroughly and SLAM it.

    You mention vacuuming but it does not appear pool has been vacuumed in some time......am i missing something?
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    Re: Black (I think?) Algae that will not go away - any help appreciated!

    Hi, yes I know and this is what has been so puzzling. I've literally kept chlorine above 15 for a 2 week period last season (first with liquid chlorine and then cal hypo) and it made only marginal difference. The algae would still reappear within two days. Also for probably 2/3 of last season the FC level was 8 - same story.

    Re vacuuming, see the last pic - I deliberately waited till after testing to take a before/after shot. Last season I tried brushing the algae almost every day (no exaggeration), it would always brush away very easily but be back the next day or second day without fail.

    That's why I've posted on this forum, as the pool shop here & people I've spoken to have not been helpful at all.

    Could the algae have built up resistance to the chlorine?
    IG 9200 gal / 35000L Marblesheen / Plaster Pool, 24" Hayward Pro Series S244T2 Sand Filter (235LPM), Webster/Speck 1HP Pump, Crystal Clear SWG 7V DC / 30 amp.
    TF-100 Test Kit

    Lives down under, in Brisbane, QLD, Australia!

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    Re: Black (I think?) Algae that will not go away - any help appreciated!

    Quote Originally Posted by drewh View Post
    Could the algae have built up resistance to the chlorine?
    Short answer ... no.

    You also have a misunderstanding about the SWG. Adding a little salt will not make the FC level go up ... unless the salt level was so low that the SWG was not actually turning on. If there is adequate salt for the SWG to function, then it will basically make the same amount of FC given run time and % output setting.

    Also, realize that we do not care how reputable you think the pool store is ... we generally do not trust pool store testing. They have been proven inaccurate and not repeatable over and over again. I highly recommend you invest in one of the Recommended Test Kits. Then you need to properly follow the ShockLevelAndMAINTAIN Process.
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    Re: Black (I think?) Algae that will not go away - any help appreciated!

    Understood re SWG, this was what I was told by a junior there so no doubt not correct. I've read up & know quite a bit about pool chemicals, pros/cons of each type for the pool surface etc.

    I can completely see your point re pool shop testing & have read those exact words many times over in pool forums. I just mentioned that as it is a reputable place, that's all. Just forget I mentioned that as well.

    I will look at getting that test kit, have just been trying to keep costs lower as inevitably I'll need to add whatever stuff.

    I just want to rid the pool of this algae. Clearly something is different if keeping the chlorine high, adding copper algaecide, lowering pH etc. have all had little or no effect on it.

    Could there be anything that could survive & grow like this? Would the fact that there are rough patches or that the CH has almost always been very high have anything to do with it?
    IG 9200 gal / 35000L Marblesheen / Plaster Pool, 24" Hayward Pro Series S244T2 Sand Filter (235LPM), Webster/Speck 1HP Pump, Crystal Clear SWG 7V DC / 30 amp.
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    Lives down under, in Brisbane, QLD, Australia!

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    Re: Black (I think) Algae that will not go away

    Just for reference, this is how the pool looks only 2 days after completely vacc'ing it (and it being perfectly clear after it) - the algae is darker and worse than what appears in the pic:

    IG 9200 gal / 35000L Marblesheen / Plaster Pool, 24" Hayward Pro Series S244T2 Sand Filter (235LPM), Webster/Speck 1HP Pump, Crystal Clear SWG 7V DC / 30 amp.
    TF-100 Test Kit

    Lives down under, in Brisbane, QLD, Australia!

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    Re: Black (I think) Algae that will not go away

    I've never seen black algae, so will not venture any opinions on identification of what is shown in your photos.

    However, reading through this thread, it strikes me that the unsightly phenomenon in the pool is stated to be algae, even though it hasn't really been identified as algae. The fact that it doesn't seem to respond to high chlorine levels would probably argue against it being algae.

    Have you tried 'capturing' some of the stuff in a container? Maybe see if it continues to grow? See how it reacts to chemicals added to the water? Perhaps look at it under a microscope (if you or someone you know has access to a microscope)? Rub it between your fingers to see what it feels like?

    Just saying -- you've already put a LOT of time and effort into fighting this stuff; maybe it's worth putting some effort into direct identification.
    18' x 48" ring top pool (Summer Escapes); 5500 gallons; set up June - October, stored during winter; Intex 2500 gph pump (B size cartridge filter) Hayward 21" sand filter + 1.5 hp single speed Powerflo Matrix pump (upgrade October 2016) *** K-2006 test kit, refills from tftestkits

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    Re: Black (I think) Algae that will not go away

    If you buy one of the recommended test kits you will save money in the long run, as you will buy only the chemicals your pool needs instead of guessing and tossing stuff in to see what works. I have a couple of questions for you.
    1. Have you tried crushing some vitamin C tablets, putting them in a sock and leaving it on the stained area? If not try that and leave it for several hours. If it lightens the stain then you will need to do an ascorbic acid treatment to get rid of it.
    2. You say the plaster is rough and pitted in some areas. How old is the plaster?
    3. Does your pool have a skimmer? You have a lot of plants very close to the pool that appear to continually drop debris in the water. The tannins in some plant materials can stain the plaster, although high levels of chlorine will usually take care of the stain over time. Adding the poolskim or similar device will help collect the floating debris before it drops to the floor of the pool.
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    Re: Black (I think) Algae that will not go away

    Quote Originally Posted by singingpond View Post
    I've never seen black algae, so will not venture any opinions on identification of what is shown in your photos.

    However, reading through this thread, it strikes me that the unsightly phenomenon in the pool is stated to be algae, even though it hasn't really been identified as algae. The fact that it doesn't seem to respond to high chlorine levels would probably argue against it being algae.

    Have you tried 'capturing' some of the stuff in a container? Maybe see if it continues to grow? See how it reacts to chemicals added to the water? Perhaps look at it under a microscope (if you or someone you know has access to a microscope)? Rub it between your fingers to see what it feels like?

    Just saying -- you've already put a LOT of time and effort into fighting this stuff; maybe it's worth putting some effort into direct identification.
    Really appreciate everyones help & advice here - thanks so much for taking the time to help out!

    singingpond you are 100% correct - I've just assumed it is an algae / organic growth as it seems to come back in more or less the same areas (though that changes over time), though it is black in colour and I know actual black spot algae is usually small tight spots that doesnt brush away at all (this is almost like a powder in the ease of which it brushes away). I've often wondered if its something else hence asking if there could be something chlorine resistant or if the slightly rusted bolt on the handrail had something to do with it (tested for iron and it was 0). I haven't got into the pool yet to try and take a sample but I will do so - if I left it in the container would it keep growing or? I'll try rubbing it between my fingers to see what texture it has.

    The only interesting thing is this same coloured powder / algae is around the two actual black spots in the deepend (see pics in an above post), which makes me think its somehow related to those..

    I also notice the shallow and deep end pool steps get covered in the same stuff quite quickly - when I brush it away there is usually quite a bit on them always.

    It wouldn't have something to do with the sand filter would it i.e. this black powder is too small to be filtered so is being distributed back into the pool? I didn't think so as its always very similar patches and areas, not randomly dispersed.
    IG 9200 gal / 35000L Marblesheen / Plaster Pool, 24" Hayward Pro Series S244T2 Sand Filter (235LPM), Webster/Speck 1HP Pump, Crystal Clear SWG 7V DC / 30 amp.
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    Re: Black (I think) Algae that will not go away

    Quote Originally Posted by zea3 View Post
    If you buy one of the recommended test kits you will save money in the long run, as you will buy only the chemicals your pool needs instead of guessing and tossing stuff in to see what works. I have a couple of questions for you.
    1. Have you tried crushing some vitamin C tablets, putting them in a sock and leaving it on the stained area? If not try that and leave it for several hours. If it lightens the stain then you will need to do an ascorbic acid treatment to get rid of it.
    2. You say the plaster is rough and pitted in some areas. How old is the plaster?
    3. Does your pool have a skimmer? You have a lot of plants very close to the pool that appear to continually drop debris in the water. The tannins in some plant materials can stain the plaster, although high levels of chlorine will usually take care of the stain over time. Adding the poolskim or similar device will help collect the floating debris before it drops to the floor of the pool.
    Thanks zea3, my main reason for not looking to buy a test kit was being in Aus, the postage along was almost the price of the test kits! But you are absolutely right, if I get one then I can accurately test myself & save a heap on chemicals anyway (only a month ago I had to buy phosphate starver because the phosphates were above 6.. interestingly any other way to get that down without buying that? i.e. As part of the SLAMing and raising the FC it would take care of it?). Will look on here and online to see if anywhere here in Aus has any of the recommended test kits, cross your fingers!

    To your questions, answers below..

    1. Re Vit C on marks - No I haven't, but the areas aren't stains - see that last pic with pretty much clear blue water? this was after vaccing one section. Basically all the algae/whatever it is is removed with next to nothing left behind. Would the VitC only be for actual stains e.g. like a copper stain?
    2. Re Plaster - the pool itself is about 20 years old, but has been kept in very good condition (no cracks or obvious deterioration on the plaster. When I say pitted/rough, I mean only very slightly and more to look at versus the feel. But I have noticed some of this black algae/whatever it is will settle around those spots in particular (but in other areas also).
    3. Re skimmer, you mean like a pool cover? No not usually though I try to remove leaves etc. often. The trees around it aren't an issue so much as a large neighbours tree that drops leaves in - but its easy enough to remove them each week etc.

    Would the CH being normally over 400+ (assuming their testing is accurate) in every test have anything to do with causing this? Sometimes the CH reading reads 800 because its over whatever maximum the test can go to i.e. 500 or around that. The SWG gets calcium building up on the plates quite often.
    IG 9200 gal / 35000L Marblesheen / Plaster Pool, 24" Hayward Pro Series S244T2 Sand Filter (235LPM), Webster/Speck 1HP Pump, Crystal Clear SWG 7V DC / 30 amp.
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    Re: Black (I think) Algae that will not go away

    Your calcium numbers are suspect, because the only way to lower calcium is to remove or replace water, either through backwash or drain/refill, and then adding water with lower calcium level. Evaporation does not remove calcium, just concentrates in the remaining water until you refill and dilute it again, in which case it would change only due to the difference in calcium in your pool vs. your fill water. For your calcium level to be 400+, then sometimes over 600, and now down to 238, tells me you have a testing error. A decrease from 600 to 240 would require a 60% water change with 0ppm fill water. Doesn't fix the black problem, but just that the calcium numbers don't indicate anything to me because I don't think they can be correct. I have high calcium so test with the TFT kit, and they go up, but not down unless I do something to make that happen.
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    Re: Black (I think?) Algae that will not go away - any help appreciated!

    Hi Sandra, hmm that's very interesting. The only thing I have done is fill the pool up & once or twice forget to turn the tap off for a bit longer so it fills a lot more, but per your example this would get nowhere near 60% of the pool at 0ppm water! The last test was 238, before that 800 (over 500 so defaulted to max 800, before that 353 and 450 before that.

    So definitely looks more and more the pool shop test results are garbage!

    I've found a way to get the TF-100 kit here via a forwarder so hopefully will have it in a month and can do actual accurate testing.

    Hopefully the SLAMing will remove the black algae/'whatever it is'. I just wonder though, I took the chlorine level up to 24 last season for 3-4 days (this is going on the level in the test which was about 8 and calculating the amount to raise it 16ppm as I know my exact pool volume, then adding liquid chlorine which I know is 12.5% strength etc). and it made next to no difference, still back in a day or two.

    Learning so much from this forum, fantastic stuff.

    - - - Updated - - -

    Would it be worth my vaccing to waste as much of the black stuff as I can (quickly so I don't lose too much water!) and see what happens i.e. what returns? I might do that this afternoon then top up the water lost.
    IG 9200 gal / 35000L Marblesheen / Plaster Pool, 24" Hayward Pro Series S244T2 Sand Filter (235LPM), Webster/Speck 1HP Pump, Crystal Clear SWG 7V DC / 30 amp.
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    Re: Black (I think?) Algae that will not go away - any help appreciated!

    Quote Originally Posted by drewh View Post
    ...
    So definitely looks more and more the pool shop test results are garbage!

    I've found a way to get the TF-100 kit here via a forwarder so hopefully will have it in a month and can do actual accurate testing.
    Yes, it will be better to have reliable test results -- congrats on figuring out a way to get a good test kit into your hands!

    Hopefully the SLAMing will remove the black algae/'whatever it is'. I just wonder though, I took the chlorine level up to 24 last season for 3-4 days (this is going on the level in the test which was about 8 and calculating the amount to raise it 16ppm as I know my exact pool volume, then adding liquid chlorine which I know is 12.5% strength etc). and it made next to no difference, still back in a day or two.
    Were you doing this on a daily basis -- i.e. testing, finding chlorine levels to be around 8 ppm, and then adding enough to go up to 24? If you've been reading about the SLAM process here on the site, you've probably realized that you were not maintaining shock chlorine levels with that approach. In other words, if your CYA was 60 (another uncertain data point, as the CYA test was presumably done by a pool store), you would have to maintain FC of at least 24. Getting up to 24 once a day, and letting it drop back down to 8 by the next day, would just be an inconvenience to any algae in the pool. The algae would have plenty of time to bounce back before the next daily dose.

    Would it be worth my vaccing to waste as much of the black stuff as I can (quickly so I don't lose too much water!) and see what happens i.e. what returns? I might do that this afternoon then top up the water lost.
    This would help if you have a filter problem. It probably is worthwhile, as an experiment, while you wait for your new test kit to arrive. Unless you are under severe water restriction, I wouldn't vacuum quickly, though -- that would tend to stir up the black stuff, and disperse it back into the water. Take your time, and sneak up on it, so you get as much of it out of the pool as possible. When was the last time you looked inside your sand filter? How often do you backwash?
    18' x 48" ring top pool (Summer Escapes); 5500 gallons; set up June - October, stored during winter; Intex 2500 gph pump (B size cartridge filter) Hayward 21" sand filter + 1.5 hp single speed Powerflo Matrix pump (upgrade October 2016) *** K-2006 test kit, refills from tftestkits

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    Re: Black (I think?) Algae that will not go away - any help appreciated!

    Quote Originally Posted by singingpond View Post
    Yes, it will be better to have reliable test results -- congrats on figuring out a way to get a good test kit into your hands!
    I so agree! Looking forward to using it & getting PROPER results will cost about AUD$130/40 (ouch) but will definitely be worth it in the long run and all the chemicals saved.

    Were you doing this on a daily basis -- i.e. testing, finding chlorine levels to be around 8 ppm, and then adding enough to go up to 24? If you've been reading about the SLAM process here on the site, you've probably realized that you were not maintaining shock chlorine levels with that approach. In other words, if your CYA was 60 (another uncertain data point, as the CYA test was presumably done by a pool store), you would have to maintain FC of at least 24. Getting up to 24 once a day, and letting it drop back down to 8 by the next day, would just be an inconvenience to any algae in the pool. The algae would have plenty of time to bounce back before the next daily dose.
    No I wasn't, you're 100% right. Part of the issue was the pool shop couldnt actually test above 8ppm! Only could tell if over 15 as it bleached the tablets. So on chemistry quantities I got it to 24ppm (assuming the 8 was accurate) but it didn't stay there. I think their CYA test was reasonable as the test to test results were very similar, I was testing the water weekly.

    This would help if you have a filter problem. It probably is worthwhile, as an experiment, while you wait for your new test kit to arrive. Unless you are under severe water restriction, I wouldn't vacuum quickly, though -- that would tend to stir up the black stuff, and disperse it back into the water. Take your time, and sneak up on it, so you get as much of it out of the pool as possible. When was the last time you looked inside your sand filter? How often do you backwash?
    Great, I will do that tomorrow or day after. To be honest I haven't looked inside since it was installed! I should do that this weekend also. I usually backwash every 4-6 weeks, when the pressure increases 2 psi above the normal 10 psi operating pressure. The fact the black algae/whatever comes back in the same spotss makes me think it isn't a filter issue but still worth looking.

    I want to try and do as much as I can until the test kit comes to eliminate things. I suppose with the SLAMing it should definitely be killed regardless.

    Unrelated question - the SWG gets calcium build up on it quite quickly. I don't use any cal hypo chlorine in the pool (nothing additional to the SWG at the moment), so why is that happening? obviously the CH level is high in the pool? (but better to be higher than lower for a concrete/plaster pool to protect the plaster correct?).
    IG 9200 gal / 35000L Marblesheen / Plaster Pool, 24" Hayward Pro Series S244T2 Sand Filter (235LPM), Webster/Speck 1HP Pump, Crystal Clear SWG 7V DC / 30 amp.
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    Lives down under, in Brisbane, QLD, Australia!

  19. Back To Top    #19
    Mod Squad jblizzle's Avatar
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    Re: Black (I think?) Algae that will not go away - any help appreciated!

    High pH coupled with high CH can quickly scale up the cell. Keep the pH lower and the problem should reduce.
    Jason, TFP Moderator
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  20. Back To Top    #20

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    Re: Black (I think) Algae that will not go away

    If you are able to scrape off some of the black spot and then smear it on a white piece of paper, if it's green (or dark green), then that is black algae. If not, then it may be something else such as an organic stain. If it doesn't scrape off, it may be a metal stain.
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
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