Source of Algae

I have had a running war with mustard algae for the last couple of years. Every time I get it under control and think I won, it sneaks back into my pool.

Recently, I went to borates to clear the water and stop the large amounts of chlorine I was using trying to get the upper hand. (which worked great)

I decided to replace the burned out light bulb in my pool light and discovered the home of my algae problems. Behind the light, in the recess was several hand fulls = to a quart or more, of mustard algae residue. Enough that would be able to protect some live spores to restart the growth. When I pulled the light out, it shot out and clouded the water. I grabbed my vacuum hose and vacuumed to waste what was left in the housing, there was still a lot there. Then upped my FC to shock levels.
This morning, the water is clear and the bottom of the pool is covered with the residue that got out into the water, it will soon be vacuumed to waste.

I guess my reason for posting this, is I spent a lot of time and effort trying to rid my pool of mustard algae. Including soaking everything that went into the pool (more than once) in buckets of 25% chlorine and water. Yet, I missed one spot and based on the amount of residue in that housing, the main reason for my reoccurring algae problem.

Thanks for letting us know the source of your recurring algae. Wow, this stuff is pretty hardy -- I don't suppose there is very much light that gets into the recess behind the light, but possibly just enough to keep it barely alive (clearly, water gets into that niche). This helps explain why just doing the shocking (at higher FC levels than normal shock -- 60% of CYA level after first lowering pH to 7.2) would kill an existing bloom but those with mustard/yellow algae would need to keep a higher FC level (15% of CYA) to keep it away and couldn't use Ben's chart values ("Min" being roughly 7.5% of CYA) or else it would keep coming back.

So when this source of algae finds a niche to live in, it takes a permanent algaecide to keep it away -- either higher levels of chlorine, Borates, and I suspect other algaecides would work as well such as PolyQuat 60. That's also very good information about the Borates keeping the yellow/mustard algae away. I think that's the first report of it being successful against this algae. Is your pool at 50 ppm Borates or some other level (since the recommendation is 30-50)?

Richard

I ended up at 80ppm of borates, either my pool is smaller than I calculated or the 2 boxes I put in when I opened it didn't dilute as much as I expected. More likely it's a combination of both. It's a freeform pool and only has 1 side that's straight, it's has curves around the rest with 4 steps in one corner and about 20 total feet of seating. I calcu-guessed 20,000 gallons, but it may be as small as 16,000.

Anyway, after vacuuming to waste this morning, the pool looks as good as it ever has. I haven't found a 300w floodlight bulb yet, so the light housing is still sitting on the deck.

It may be the higher borate level was the reason for the results against the mustard algae. But, I'm no chemist, you guys lose me at times.