i've got a sera 50w in my aquaclear 300... and a piece of plexiglass glued in with some liverock on the other side. the plexi keeps the current out of the tiny refugium.
also with the aquaclears (except for the 500 and the mini i think) you can swap the impellors to reduce the current... i've got a 150 impellor in mine. works great, nice to have the heater outside the tank.