I beg to differ

[dschlic1]

Recently I posted a possible reason for changes in the stabilizer levels in pools that are turbid. Here is my post:

Another possible reason for the differences in the readings is due to the "algae" eating the stabilizer. Typical scenario: Pool owner finds a white turbid pool, checks CYA to find shock level. CYA measures 40 ppm. Shocks the pool, meanwhile the bugs are munching away on the CYA. Poll finally cleans up, pool owner checks CYA again. Finds the new value at 20 ppm. Probably 15 ppm loss was due to the bugs

Waterbear then said in reply:

Algae does NOT eat CYA. CYA is consumed by anerobic (oxygen hating) denitrifying bacteria that are commonly found in soil. For this to work the pool MUST be stagnant because any circulation would increase the oxygen and the bacteria would die! This is why CYA sometimes disappears in a closed pool during the winter. The water is NOT circulating and is stagnant so the aerobic bacteria can grow. Also, this is a slow process and does not happen in the matter of just a week or two.

The effect you are talking about above is from the clouding of the water from the algae and it's interference with the CYA test!

I rest my case!

Well Mr. Waterbear I beg to differ with you. I have had the exact scenario I described happen to me. And my pool was NEVER stagnant. I agree that the organism(s) in question are probably not algae. That is why I place that word in quotes. And if it happened in my pool it can happen in another.

[JasonLion]

There have been three or four reported cases of CYA disappearing this season with no obvious cause, and small numbers of similar incidents have been reported in other years.

The three most common causes of CYA disappearing during the season are: testing error, taking the initial reading when the water is murky and thus getting a higher reading than the actual level, and water replacement lowering the CYA level (typically from a leak combined with an autofill system).

However, even taking the know causes into account, there are still a few incidents each year where the drop in CYA level can not be accounted for. While these unaccountable incidents appear to be related to algae in some way, it isn't obvious how they actually happen.

[ivyleager]

I had used pucks extensively initially with my pool. After 1.5 seasons, the CYA was reading 100. I am on a well, and while I filled my pool from it and currently top it off, I don't relish dumping and refilling. I got used to using a lot of chlorine at the end of the second season when I found PoolForum. That winter, I did not officially close my pool. Turned the pump on when temps got low over the winter, which isn't very frequent here in NC. By late spring, I neglected adding chlorine to the pool as soon as I should have and had an slight algae episode. Didn't take more than 4 days to clear. Once I turned it around and retested chemicals, my CYA was 30ppm.

Yes, the pool was stagnant, except for a handfull of days when temps dipped to freezing. No, there was no debris in the pool as I kept the leaves and crud out that whole off season. Scotts Lawn service had been around prior to the algae bloom, but they blow off any products that get on the pavers and decking. Could some of their fertilizer have acted as an anaerobic agent?