well, from what i have read and experienced myself it's a little warm but as long as you turn the light off sometimes and let the water cool down i dont see a problem with it they like the temp to be around 78-80 degrees anywway, it really boosts their immune system. i have a small jar for one of my betta's and i have him on top of my tv where he gets nice an toasty! and he loves it! so as long as they are allowed to cool down sometimes i dont think there is a problem!
Good luck and enjoy