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.