I have a large power head behind my rocks that's tee'd off.
Originally i had it on a timer, but then I saw my pseudochromis sneak into the outflow tube when it was off... i decide to take it off the timer
anyone ever have that happen to them? (ie with a dart fish or something small?)
A closed loop is just for flow. Usually you will have the tank drilled for a closed loop but you can go over the side and use a manifold system. Baiscally it is just pvc to take water out of the tank and through a pump and back into the tank. The only opening is the tank itself so it is just used for circulation not filtration.
I have a mag drive (tough little buggers) and a Seaswirl with loc-lines to direct the flow. I get nice movement out of "me little lovelies" aka corals!