Green water

Question
Hello, My tank is making me mad, in a way. I have a 40 gal. tank and 4 fish. I have a small Koi, two common 'Comet' gold fish and a plecaustomus (or however its spelled) I change my water every week and I do about a 75% change when i do it, I even dig in the rocks and un-earth all of the hiding grossness. I have a filter thats the recommended size for the tank... but.. Its never clear water. On occasion I can see almost through the tank. I clean it and it stays cloudy for 3 days and immediately goes back to this nasty, neon green. Why?

Answer
Hi Valerie,
Green Water usually indicates too much light.  If your aquarium is near a window where it is getting sunlight, this could do it, or if you leave your aquarium lights on too long.  This is the only reason I know why aquarium water would turn green.
Lynda