I know I agree with you, it just started one day and looks terrible! This tank has zero ammonia, nitrite or nitrate readings what so ever. It's a aquaponics tank (plant filtration, no filter). I do WC's for mineral replenishment only (every other week about 20%). Do you think that might be contributing to the problem?
More info: pH 7.6, 65-72 degrees (cold water tank, no heater), Marineland LED's on for 8 hour per day period. The Tank gets direct sunlight but no green water or algae issues. Circulation is Eheim 2211 canister with no media, only sponges. I have not tested gH/kH or TDS. Tanks been running 6+ months.