Report Green water in swimming pool in All Swimming Pools Types Posted June 27 22 minutes ago, dswanson said: As for why it's happening in the first place - it's due to not having the right chlorine level for the cyanuric acid in your pool water. Once that is set you can remove phosphates to make it harder for algae to reproduce. So much wrong here. Algae blooms are not always caused by running chlorine too low for the CYA level. Also, FWIW, this information on CYA was first published by Ben Powell on Pool Forum (and was known since the late 60s because of the research carried out by John A. Wojtowicz of Chemcon but suppressed by the largest manufacturer of stabilized chlorine) and carried over to TFP, as was the BBB method which was also developed by Ben Powell. I as one of the original Mods on TFP when it first started and actually wrote much of pool school. However, when the board was sold to it's current owner there were some issues so I left and they removed my name from what I wrote but many of my posts are still there. The info on Borate is based on work that I did along with Chemgeek originally on Pool Forum and later carried over to TFP. Now, as far as phosphates go, this is just a moneymaker for pool stores. Algae nutrients are phosphate and nitrate. Phosphate is testable and Lanthanum salts will cause it to precipitate out (and cloud the pool and and clog the filter, this is how phosphate removers work). Nitrates are also testable. However, the only way to remove them is by draining and refilling with nitrate free water. Nothing for a pool store to sell you so no money to be made. Phosphates commonly enter pools by fertilizer runoff. Nitrates commonly enter pools either by runoff of fertilizer or sweat, feces, and urine, which EVERY bather adds, no matter how clean they think they are. Animal droppings are also a source. Now, as far as algae blooms go both nitrates and phosphates are algae food. Nitrates are more often the limiting factor in algae growth than phosphates so in most cases only removing phosphates has not effect. IF phosphate is the limiting factor then they do work but they are messy to use. A much better solution, IMHO, is to add 50 ppm borate for it's algaestatic properties and maintain proper FC for the current CYA level and shock to the proper level (which is all the SLAM procedure is. The reason TFP created the word SLAM was to end confusion on the fact that shock is a verb, not a noun. It's something you do by raising the FC and keeping it there until algae is killed and not adding a product called shock. Products called shock are nothing more than chlorine or MPS. Chlorine will kill algae, MPS won't but can help with persistant chloramines in certain cases) Hope this clears things up.