I haven't posted here for a couple of years.... thought I would chime in, though.
I've done two things in the past to add extra light to my 100 gal
: One was to remove the cover from part of the tank and use an old light fixture from a previous tank in addition to the one I was using. The double light fixtures helped a lot. If you don't have an old one around, they also make some "clip on" type light fixtures that aren't that expensive, which could be used to boost your light - you can get a reasonable size for under $100. The other thing I did was to buy some "under water LED
lights" off of ebay. They attach with suction sups in a row at the top edge of the tank. The downside to these is that the glass on the lights needs to be cleaned periodically, coralline scraped off, etc. They wouldn't be enough on their own.... but they are supposedly good on energy consumption, and they did give me enough extra wattage, i.e. there is a noticeable difference when these lights are off and I am only using my power compacts. Here is one version of these on ebay currently:
28" 60 white Wide LED Bar car Aquarium Lighting Power - eBay (item 160442349673 end time Jun-13-10 08:48:20 PDT)
Mine have actinics mixed in with the white lights, but there are many types out there.