I've been setting up a planted 20g long, have had my four corydoras for at least three weeks. Today i noticed that one of the corys has a hairlike, moldy growth over his mouth. He is also lethargic, and surfaces for air much more frequently than any other fish in the tank...
Starting with a 20 gallon long glass tank (24"L x 16"H x 12"W)
Rio Internal Filter 400
None (Previous one broke)
Big Boy Single Bulb Aluminum Lump Casing &
15 watt Extra Day Hi-Glow Bulb
2 T8 18 watt floursecent bulbs.