Is the compact fluorescent you have spiral shaped, per chance?
The "equivalent to 50W" label that the bulb has on it is misleading. They're trying to say there that the bulb will light the same area as a 50W incandescent does. That doesn't mean it's more intense lighting. For aquarium use, you'd only count the 11W or whatever.
At Walmart you can go to the fish section and find their screw-in mini compact fluorescent lights. They're 10W but they're somewhat longish bulbs and not spiral, and will do a good job in a 10-20 gallon tank. I think they're around $5 each. I would replace your incandescent with one for sure, and if your other screw-in is a spiral shaped bulb, or one of those non-glass "diffused light" type bulbs, I would replace it as well.
http://www.aquajake.com/blog/hardwar...-compact-bulb/ is the one at Walmart I'm talking about, found in the fish section.