I have a 55 gallon fish tank with 2 very large goldfish (6 in from tip to tail). Three weeks ago I started to have green water after I did a 25% water change. I t came on very fast and any algae killer I put in was a waste of time. After a week I did a 50% water change and the same thing has happened. It is not on the tank it is in the water. If I take the glass top off it drips green water. I have just tested my water and these are the readings:

GH -30
KH - 40
PH -6.5-7.0
NO2- 0.5
NO3-a light 40

My question is what is wrong with my water? It came on very suddenly and I have never had anything like this before. Another thing is that the temperature is 70 and the light is a year and a half old and has two 17 watt aquarium bulbs. Also the fish don't seem to be adversely affected by it. Thank you for any help.

My tanks grow algae very fast also. Everything I have been told is that if the algae is healthy than the water must be healthy. I have found that chinese algae eaters are very effective at getting rid of the algae. They can grow very fast and are very active fish they have done a great job at keeping the algae levels down in every aquarium I have