Black Algae is taking over my FG IG pool, HELP


New member
Aug 10, 2008
I have an IG Fibreglass pool, 25,000 gal (my sig says 20K-wrong), Chlorine, test with the Taylor DPD-FAS kit, Sand filter, Chlorine feeder, Polaris pool sweep. NO heater. We use well water (tempered with Metal Free), surrounded by oak trees and granite rocks with moss and lichens on them. Maintain FC = 4-5, CC = 0, PH = 7.8, CYA = 40. Phosphate is OK.

Have a 2 level pump, running at low 24/7. Run on high with pool sweep for 6 hours every other day. Brush 2X week.

After 2 years of struggle and tremendous instruction and help from TFP, I am finally in control of the ordinary green algae that used to take over my pool

Last year developed about 30 black algae spots in various areas, which I was able to get off with a chlorine tablet and a brush but they came back. In fall, we drained the pool and did chlorine wash. That worked. Pool was beautiful all winter. Suddenly now the black spots are back with a vengeance, multiplying daily. We have a polka dot pool. They do not respond to anything. It is black algae, I can scrape some of it off with my fingernail and it is black/green.

I was recently told that a Chlorine/Aqua Ammonia treatment might work. Supposed to pour 1 gal Chlorine in each of 4 corners of pool and 2 gal of Aqua Ammonia (not sure what % to use-found one on-line of 4%) in center and circulate. And brush. Said it was an old remedy with pool services. Haven't been able to find anything about it on any of the forums. Does anyone know if it works. Is it dangerous? One post said Aqua Ammonia by itself will not kill algae. Will this work? Any other suggestions?

Sure would like not to have to drain pool again. If I do, what can I use to kill it forever?

Thanx for your help.

Bama Rambler

Mod Squad
TFP Expert
LifeTime Supporter
Jun 22, 2009
SouthWest Alabama
Never heard of that before. BA is hard to get rid of.

I would lower the pH to about 7.2 and keep it there and raise the FC to shock level and brush the spots with a good stiff brush until they're gone. Once they're gone, lower the FC to 7 and keep it there.


Active member
Jul 6, 2011
Chambana, Illinois
I don't understand why anybody would recommend dumping ammonia in a chlorine pool. Ammonia and chlorine produce very bad things--Chlorine gas, nitrogen trichloride, and hydrazine to name a few. There are many threads on this forum about people trying to get SMALL amounts of ammonia OUT of their pool. The answer is to add chlorine-- and it takes a lot of chlorine.

Do not add ammonia to your pool.