Algae spots, perhaps?

[Chaddy]

Here are my TF100 numbers from Sunday, two days ago:

FC - 3
CC - 0
PH - 7.5
TA - 80
CH - 400
CYA - 35
Borates - 50

The FC was a little lower than I wanted and so I bumped up the feed on the Liquidator a bit, and according to a quick drop test yesterday and a strip this morning the FC is ~5. Anyway, I have been getting some recurring spots on a seat that is right under an overhanging tree branch. The spots are usually about the size of a dime or smaller and look like rust at first glance. They can be brushed off with a metal brush. If not brushed, they will go away during the day and then reappear overnight. The water is crystal clear (I recall someone posting that they could see if a quarter was heads or tails at the bottom of their pool, well that's pretty much how clear my water is too). I haven't carefully checked if I am losing chlorine overnight yet. Pump runs on high 8 hrs a day and low for another 12. It shuts of completely from 10pm to 2am. We did have a couple of inches of rain last week. I haven't gone to Leslie's in a long time, but I recall phosphates being high the last time I went (maybe a year ago). Since I am having a party this weekend I went ahead and added bleach to bring it up to 14, although I am not sure if it got there because I had to get to work. I will check carefully tonight. Just wondering if anyone has seen anything like this and if there is any advice? I have never had to shock before and I wasn't really sure if I should have now.

Picture is from last June before I started using the TFP methods, but this is what the spots look like.

 

[frustratedpoolmom]

The "reappearing" overnight is odd. Does this correlate somehow to a reduction in chlorine levels?

Black Algae, when brushed, the spots still appear, then they fade once the roots are exposed to chlorine. We recommend brushing because it knocks the tough chlorine resistant outer layer off, exposing the roots. But you still see the spots, lighter at first... until you continue to brush and they eventually fade away. Is this what you're experiencing? Or the brush away instantly? If it brushes away instantly - it's dust/dirt/dead algae, and the circulation being off for a short time over night would explain why they appear overnight.

Phosphates are essentially irrelevant if you are diligent following the methods here and maintain proper chlorine levels. Pool Stores make a big deal out of phosphates because they can sell expensive magic bottles that will empty your pockets.

 

[Butterfly]

Sounds like tree droppings that fade w/chlorine.

 

[Chaddy]

Poolmom - I agree, I don't get why they come back overnight. As far as I can tell I am not losing chlorine overnight, but I admit I have not checked carefully. I usually just do a quick check in the morning with a strip before I go to work and then do a drop test in the evening. I have not noticed anything out of the ordinary. Again, according to the strips, which I have compared to TF100 readings in the past, I was at 5 this morning. I have been able to brush these off in the mornings, but it does not come off easily like dust or dirt, you need to put a little elbow grease into it, but I am able to get them off completely by brushing for 10-15 seconds with a steel bristled brush. I didn't think phosphates should be a problem either, but these spots have me a bit confused. Well, I am in the middle of a shock now, so I guess I will see what happens. Does the spot in the picture I posted look like black algae? Thanks for your help.

- - - Updated - - -

Butterfly - Yeah, I thought about that too. Maybe I will prune that branch back. I'd rather not though. What would a tree dropping be? Would it be resistant to brushing away with a metal brush? Would it look like the picture I posted? What I might try is to put a cover over that seat and see if the spots come back. I'll have to wait for this shock cycle first. Thanks for the idea.

 

[Chaddy]

So I got home. Spots are gone. It's been 85 degrees and sunny. FC = 9 and CC = 0. Liquidator has been on and usually maintains FC at about 4. The amount of bleach I added this morning should have raised the FC to about 14, but this is just a guess since I did not measure it. I will do an ONCL tonight to see what happens. Any other suggestions?

 

[Chaddy]

OK, so I am still confused, although I am guessing it is "tree droppings". So my ONCL was zero. FC = 8 and CC = 0 at 8:30 last night and the same readings at 6:00 this morning. However, the spots were back. I scraped them off with a brush this time. If it's not tree droppings, could it be inorganic? If it's inorganic, why would they go away during the day? If it is tree droppings, why would they be able to form in a FC environment of 8 with CYA 35? Could my CYA readings be off? I am going to cover the area tonight to see what happens. Any other thoughts would be appreciated.

 

[frustratedpoolmom]

It could be black algae. Next time you get one, scrape it with your fingernail - then rub the substance on a white sheet of paper and see if its green.

When you did the OCLT was the liquidator off, and the filter running 24/7?

From your descriptions, I'm suspecting Black Algae. You're probably going to need to brush 2-3 times a day for a few days and keep the FC up. I would suggest 2 things... since your CYA is only 35 - once this is defeated I'd raise to 50 since you have the Liquidator. I would also suggest running the pump on low 24/7 - You really only need to run it on high for vacuuming, etc. You'll save more on your elec bill doing this than your current schedule. When there is no filtration - FC "dead spots" can occur - this has been documented on the forum before. If the roots aren't really killed off, and the filtration stops, the spots can return. I do think you have a BA situation here.

 

[Chaddy]

Will do with the fingernail test. The Liquidator was off, but the pump was running as per schedule. Low from the 8:30pm reading until 10pm, off until 2am, and low from 2-6am when the second reading was taken. If I want to go low 24/7 then I might have to adjust the rate. I am not sure the current low setting will cycle the water once per day. I will look into this and try it out. I will definitely take out the dead period from 10-2. I did not sample the water over the spots for the tests, so it may well be a dead spot. Thanks for the help.

 

[Chaddy]

Just an update in case anyone was following this. I allowed my pump to stay on low throughout the night and the spots have not returned. I have not kept the FC levels high but the Liquidator has been adding some chlorine, and I have not brushed much. I know, I am probably asking for a return appearance. Last two days FC has gone from 8 to 7 and 7 to 6 with no CCs each time I have tested. One other little wrench that was thrown into this is that my pool builder had to replace our UV system. It had been off for several months due to a potential fire hazard and two days ago, when I also kept the pump running on low overnight, they installed the new unit. My guess is the improved circulation is what did the trick since algae (if that is what it is) stuck to the seat is not going through the UV, but this has been far from a controlled experiment. I am now going to experiment with cutting back the time that the pump is on high, while keeping the low setting on throughout the night. Thanks again for the advice.