Can you help with a cloudy freshwater aquarium?

Ok, I have a 10 gallon freshwater aquarium, not too many fish, using the appropriate size filter, heater, etc. The tank was crystal clear for the first 2 months, then in the past week has gone really green cloudy. The temperature is fine, I changed out 3 gallons of water a week ago, then again 2 gallons tonight. Using the anti-cloudy drops from the fish store and everything. Any suggestions?