I have one tank of mantellas, and since they like it a little cooler than darts, I had the same problem. I don't use any heat sources besides the light for the darts. Just sitting on the tank makes it warmer. So what I did for the mantellas is I have them on a shelf and the light is attatched to the shelf above about 2 inches off the tank. That makes it cooler for them because the light isn't directly on the tank. There may be a better way but I thought I'd post since no one else has. Hope it helps.