My brother has about 5 gourami's. His tank got this very foggy green which looked like algae. So I cleaned the tank for him .. .and THE NEXT DAY IT WAS BACK! What could it be ?

Is the tank near a window where it gets a lot of sunlight? Does he leave the tank light on? It just sounds like an algae bloom, which is nothing really to worry about. If the tank is cycling then it could just be the bacterial bloom which is perfectly normal in a new tank. Try leaving the light on less or closing a window to lessen the light. Also test your nitrates. If theyre high (over 40) then that could be contributing to the algae problem too. Also when you say you cleaned the tank you mean just a water change right, not the whole thing?

Well we kept most of the water. We took the fish out and put them in a pail and cleaned the actual tank with water only. So there was about a 40 - 50% water change.. We only did it because of the algae problems. His tank is right by the window and he keeps his light on ALL day. So js this is most likely the problem ?

Yep, just an algae bloom!! Nitrates combined with warm water combined with lots of light equals… tons of algae! LOL If you can keep the window partly closed or block the tank from the window (I used to keep cardboard behind my tank when it was in the window) should really help. You can also do water changes and like I said keep your nitrates low. Also only leave the tank light on about 8 hours a day unless you have plants in there. And this isnt harmful to your fish, it just looks nasty. Boy with the advice Im giving you would sure laugh if you could see my tanks right now! Good luck.

LOL I was shocked js. I has just cleaned it a day and a half ago and bam it jumped bacon on me. I will have to educate my bro, although I do not know much about tropicals