If your question was more of a concern about heat buildup, I would presume that if you remove the lid from the aquarium, the heat wouldn't be too much of a problem - worst case, a clamp-on fan should be adequate. Alternatively, you could run a small chiller - while not inexpensive, it would help keep the evaporation loss down a little bit.
I have heard that you can go to the hardware store and get some screening material like the screen for windows. You can get that and layer it or cut it to fit the tank to help diffuse the light. Then you can remove layers as the tank inhabitants adjust. That way you can keep the photoperiod the same and just diffuse the light to reduce intensity. Raising the light also helps, but not always feasible unless your light is mounted above the tank and can be easily adusted.