I can't take all the credit, I ran across this info while researching for my own bait tank. These guys were using the hobby speed control though, so finding the "fan speed control" was a great alternative. Its already set up for 12 volt input.. Check out the pic in the link. Notice the little dial that adjusts voltage;
http://img.tomshardware.com/us/2003/..._fanmate_1.jpg
This is very similar to the one I found at Fry's