Printer paper would work, probably will not catch on fire. Would produce a diffuse light. Remember Farenheit 451? 451F is the flashpoint of paper.
A few thin strips of aluminum foil would work as well. Think small 1" by 20" long strip or two to reduce downward light. Just lay on the top of your hood under the light.
If your lights have an aluminum reflector, you can remove it unless it is acting as a heat shield. So see how hot the lamps are. If it has the aluminum reflector/heat shield, you could get some BBQ black paint (high temp stuff) and spray a little on the reflector/heat shield to make it less reflective.
You could also get an extra hunk of plexiglass/lexan/glass (non tempered) and have it bead blasted, or scuff it up with sandpaper to reduce light transmission to the tank. This will aslo diffuse the light more.
It is really easy to produce more light, and reduce output efficacy than to find the correct amount of lighting to start with.