I run a mag 3 on my 20L. The limiting factor is the rate of flow that your overflow can handle. Remember it must be screened off in some way to keep livestock out. I have the pump, from the sump, restricted to slow the flow. Place a valve somewhere in the return line.

I have seen some flames on this site aimed at those of us putting sumps and fuges on small tanks. I say go for it. My 20 is my favorate setup.