Have a late season outbreak of algae here. It was misdiagnosed at 1st as it appeared to be dirt/silt that had blown in the pool. It was light brownish in color. After realizing this, I brought the FC up to shock level for my CYA yesterday afternoon and ran my pool cleaner. Before I went to bed the FC level was the same and all the algae appeared to be gone from the bottom of the pool although still pretty cloudy. I removed the cleaner and left the pump/filter running overnight. Algae re-accumulated on the bottom overnight, but my FC still hasnt moved ...and I have yet to register any CCs during any of my testing which is really confusing me. What is going on here?

Test results:

FC = 20
CC = 0
pH = 7.5
TA = 60
CYA = 70
CH = 160
Salt = 2900

Are you sure it's algae? Did your FC drop below the minimum for your CYA, or is there any other reason to think its algae?

I would vacuum the bottom of your pool and as long as your FC and CC look good, assume it was something besides algae.

I'm not 100% sure its algae and if my FC level dropped below my minimum for my CYA it was ever so slightly. It was between 3-4 when I checked a few days ago when I thought it was dirt in my pool and also before I began shocking yesterday. Again, tested zero CCs each time. I was able swab a bit of the substance with my finger from the stairs and its a lightish green/brown color and whips away with the slighest movement of water. I am almost certain it is not dirt or silt. I vacuumed the pool yesterday, but alot of the offending substance reappeared. Also, my pool is cloudy. It's got to be some sort of algae, no? I am really confused here.

Do an overnight FC loss test.

I basically performed one last night. Tested it at about 10PM and it was at 20. This morning the reading was the same.

mx702,

Could you post a pic of your pool water? I think that will help get to the bottom of this.

I will grab a few shots tomorrow.

This is killing me right now. I have had absolutely zero issues with this pool all summer long and now this right before closing. I almost wish it was just an algae breakout because at least I know how to cure that.

Thanks for the replys.

When is the last time you cleaned the cartridge?

It could be pollen, I suppose. Hate to ask, but you did switch the SWG off for the OCLT?

The responses are much appreciated.



Yes, I did remember to turn off the SWG.


Just a couple days ago. When I began to notice the cloudiness I thought perhaps the filter might have been dirty even though it hadn't been too long since I cleaned it last. The filter was still relatively clean.


Last night's OCLT = 0. Here are a few shots I grabbed this morning. The debris appears to be being deposited beneath the returns as if the particulates are not being captured by my filter. Is it typical for pollen to behave in this manner?

I'm thinking that it's mustard algae, even with no chlorine loss overnight. Sometimes mustard will do that because it's very resistant to chlorine. With your CYA at 70 ppm, you're probably going to need to go to at least 28 ppm FC (40 % of the CYA) to get it under control.

Thanks for the pics. Here's my take....

It appears to me that you have a slightly murky pool with dead algae on the bottom. You only started shocking on Thursday so it is very reasonable to assume that ALMOST all the algae is dead but not filtered out of your pool yet.

I cannot explain that you have absolutely no FC loss, but it appears to me that you need to vacuum or brush up the algae and run the pump 24/7 until your pool is clear. Depending on your system, it may take a few more days but there is no other viable path that I see.

Sometimes my pool does some things that I cannot explain as well. Nevertheless, I think your pool will clear nicely in the next few days.

Just to be clear, I have not been continuously shocking. I brought the chlorine slightly above shock level (approx 22ppm) on Thursday, but have not added a drop since because when the pool was not consuming chlorine abnormally I assumed the issue was something other than algae. As of 9:30 tonight my FC level = 12.5. If it was mustard algae wouldn't I see more than 10ppm loss over the course of 2.5 days? I appreciate the advice, but I'm still unsure of my next move. Do I hope that round the clock filtration eventually clears up my pool or gear up to take on the mustard?

My experience with mustard algae is that it does not use up chlorine anywhere near as fast as green algae for several reasons. It does not grow as fast, it is very resistant to chlorine and there is usually much less quantity of it than the green. You have to go to full shock level of about 40 to 60 % of the cyanuric acid level, keep it there and brush it every day, or vacuum to waste.

Read "How to Shock your Pool" up in Pool School. You have never completed the shock process and barely got it started.

As a result, you have murky water and dead (maybe some of it is live) algae on the floor of your pool.

Shock the pool by bringing it to the suggested level in the FC/CYA chart and then finish the process exactly as described in pool school.

One dose of chlorine does not effectively shock your pool but it does produce the results you are experiencing.

Ok, took her back up to shock level this morning. My question now is: how do I conclude that the shock process is complete? I've passed the OCLT with flying colors every night (including last night), so I cannot rely on that. I'm guessing just hold shock level until the pool clears up?

Shock is complete when.....


1. Your water is crystal clear

2. You lose less than 1ppm FC on the OCLT

3. Your CC's a .5ppm or less.

One thing I have discovered about the OCLT is that you should let the filter circulate the water without the SWCG for at least one hour before making each test.

Thanks for the reply, but the SWG has been isolated for a week now.

This MA battle is really starting to wear on me. I retested my CYA level and its actually about 50. I've maintained the FC level between 17-20( shock = 16) for a couple days now while brushing the pool and running my PC the entire time to aid in circulation as I have no main drains. In the morning I still wake to find splotches of MA around the pool. Pool remains cloudy/murky and I still do not see abnormal FC consumption nor do I register any CCs. It seems others have encountered this zero CCs phenomena, also. My understanding is that MA will disappear at normal shock levels then one needs to bump the FC level up to MA shock level for 24hrs. Although I continue to see re-emergence at normal shock levels, last hour I dosed the pool's FC level up to 28. To be honest, I would unload tanker of chlorine into the pool if I wasn't concerned about my liner. Tell me this gonna get better

Algae won't reappear at shock level. If you cleaned it up while at shock level, and it came back while still at shock level, it isn't algae. Also, your photos don't look like mustard algae, but instead like dust accumulating at spots where the circulation is slightly slower and they fall out of suspension.

You have said a couple of things that make me think that something is wrong with your filter, perhaps a tear in the cartridge fabric or a bad seal, that is allowing debris to get past the filter. Debris doesn't normally collect under the returns unless it is coming from the returns, and the only way it can do that is if it is somehow getting past the filter. If you have a multi-port valve it could also be a problem with the multi-port valve, but cartridge filters rarely have multi-port valves.