If you don't see any problematic (i.e. unwanted) algae, why not leave them on as long as you want? I used to have one of my tank lights on for 14 hours a day, and fishes certainly didn't have any problem with that.Joeyt66 said:What if you dont have live plants? I keep my light on for 14 hours a day is this too much? its on a timer (65 gallon 40watt florecent bulb)i keep it on for this amount because it is kinda the main attraction in my basement, its built in the wall and lookks kinda dull if the lights are not on. Am i leaving it on to long? what is the max it should be on?