You could add an internal overflow to feed the closed loop, but I wouldn't for two reasons. One, most of the DOC
's in the tank will be at the surface and you don't want to skim this layer of water and churn it all back up through a pump...you want that going through your sump where there is a chance some of it can be removed. Second, if your sump return pump quits for some reason, the water level in the tank is going to drop as the tank drains into the sump. If you are running the closed loop from an overflow, it will run dry in this situation and likely ruin the pump. I would either come over the top edge with the suction line or drill the back of the tank and use a bulkhead fitting with a large strainer. The reason to use a large strainer is so snails, cucumbers, anemones, ect... won't be sucked in and killed. I would split the return into at least two outlets. You really want one return blowing just under the surface of the water. This will create lots of surface turbulence and increase gas exchange. It also helps cool the tank by increasing evaporation. You can use your sump return for this...be sure to drill a siphon break in that line so the water won't siphon back through the pump in the event of a pump/power failure. If you were to use a larger pressure rated pump, you could have three outlets with two of them coming off a SQWD. The third would be valved so you could control the pressure and flow through the SQWD. This would give you more than enough flow through the tank as well as some alternating flow that would simulate wave action somewhat. The target in a reef tank is turbulence. No coral is going to do well with a pump outlet blasting it 24/7. You want changing, colliding, turbulent currents that simulate the waves crashing over the reef as closely as possible. The 180 I'm installing will have one pump (1150 gph
) going into a 1" Sea Swirl which will rotate back and forth 90*. The other identical pump will go through a SQWD with outlets on opposite ends of the tank. There will be at least one 5 gal
surge device and possibly two. I guess I'm writing a book here...sorry. I get carried away sometimes. But you get the idea about the currents you need to create.