It sounds like the lights are heating the water. I had a similar prob. with my 5 gallon, I found that if I opened the cover ( the feeding area) it helped some, I also found that having a fan blowing across the tank cooled it. How hot is it getting with the lights on? The most important thing is to try to maintain a stable temp. range. If when the lights are on it is not going to high I would raise the heater a little so that when the lights go off the tank stays closer to that temp. as long as it isnt to high. I think you would be fine as long as your not going to high over 80 or having extreme swings in temps.