I Can't Get All The Algae Out!

by David

First, the conditions: I have an inground, lined pool about 36'x18' and approximately 25K gallons. The liner was new in 2010. I do not have any leaks (because my water level is consistent).

1) My pool chemistry is balanced (chlorine, pH, conditioner, alkalinity) and has been for over a month since I opened the pool.

2) I use a cartridge filter system and inserted a new cartridge at the beginning of the season (May).

3) I add a clarifier once a week to drop the solids to the bottom and make them more likely to be filtered out.

4) I vacuum about once a week and brush about once every other day.

Problem: Even after I vacuum, by the next day, there is algae on the bottom of the pool. When I push the brush along the bottom, there is a green cloud that is pushed up.

I have increased my chlorine level to about 5 to 6 ppm and shocked the pool more frequently. So what I don't understand is why I still have algae in my pool even though I clean it thoroughly and the chlorine is at an effective level?

What should I try next?

Thanks for the question David

I'd like to have your complete chemical readings:

Chlorine, CYA (cyanuric acid/stabilizer), pH, Alkalinity, Calcium Hardness, and Metals (iron and copper).
It makes troubleshooting much easier and the process of clearing up your pool will go
much faster.

You can get this done at your local pool store or use a good test kit yourself. I'd encourage you to get a Taylor FAS-DPD K-2006 pool test kit.

Pool Water Testing

Water Testing Kit

Next is to watch the clarifier use. I never used a clarifier either for the YMCA salt pool or when I had my pool route in Arizona. If the chemicals are balanced, especially with a cartridge filter, which is my personal favorite, there's no need for a clarifier.

Your algae problem could be a couple of things. Either too high or too low CYA (stabilizer) or too high pH. Anything over 8.0ppm and you're only using about 25% of the available chlorine. Add to that the wrong CYA level and you're way behind.

Even if the chlorine level is 5 - 6ppm, if either or both of the above is off, you're going to be playing catch up all the time with the algae. Shocking when you have algae is a process, not an event. The chlorine level needs to stay above 12ppm for a period of time to kill the algae.

Shock in the evening then retest in the morning (before the sun has a chance to hit the pool) and see how much the level has dropped. If it drops more than 1.0ppm, you still have an algae problem. Get me the actual chemical numbers and I'm sure I can help.


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