Both. 1/4" return as well as a 1/4" surge. I drilled a hole in my ADA "30g" for a 1" bulkhead, then made a rock structure using Emaco and MarcoRock Key Largo rubble. I left the top of the structure open and I simply rest loose rocks across it. The tank, and sump, are the only tank I have at home. It's not plumbed into any other system, but it could be if I wanted it to. No reason it won't work. I get about a 5" tidal rise and drop, though I could make it go higher. I cannot go lower, I have turtle grass limping along.