Hi, I think I just replied to a few of your questions in your other thread.
masonryx's cartridge question
Aside from that, I think the Penguin 100 is a little undersized for a 20 gallon tank. Have you considered adding another hang-on filter? It doesn't have to be the same brand. Personally I much prefer Aquaclears to Penguins, and I think the Aquaclear 20 would do a good job along with the Penguin you already have.
As far as your lighting issues, I'd suggest not using incandescent bulbs. As you've noticed, they run quite hot and can cause temperature issues. There are compact flourescent bulbs that screw into incandescent hoods that run a lot cooler, and would even give you some options for live plants if you want, even though the wattage is lower. Here's a link:
http://www.drsfostersmith.com/produc...70&pcatid=9870