you probably have your reasons, but you can buy a bio-wheel pro 60 for around 31bucks at that pet place or better yet, you can get a Penguin Bio-Wheel 125 for 15bucks there as well. It's a hob
filter with the bio wheel addition, it's rated for 20gal tanks.
i haven't tried building my own yet (have an idea of building one into a sump, like the tidepool models) but i have used bio wheels in the past and currently have a 12gal eclipse system running. In the directions for both units, it states that after the bacteria culture develops it's normal for the wheel to spin at irregular patterns, or stop for a little while, spin a little and stop again. marineland also suggests if it stops for long periods of time, to rinse the biowheel, and that will fix the problem.
well i seriously doubt a 50gph pump would be enough. stand alone biowheels use a gravity fed system and i don't think a 50gph pump would supply enough water to the biowheel to keep the gravity system running for long, i have a biowheel 60pro and when i plumbed it into my existing system [it was about 5yrs ago now], i found i didn't have enough pressure and the water would drip out of the first 2holes in the spraybar, cling to sides and run down the plastic housing and eventually back in to the tank, missing the wheel completely. probably 100 pump would do it but once you buy one of those, it will be about 10-15bucks from a LFS
, and you would save money buying the pengiun 125
hope i helped,