Ah crud... The tube coming out of the PVC
? Where one is siliconed in and the other has a brass fitting? That's to start the siphon cause I am lazy.. The turtle tank overflow will stay running after it has been started where the one for the fish tank creates such a vortex that it pulls in a ton of air so that one is used Full time to keep the siphon going strong.. Basically that's what I use the lift pumps for is to pull extra air out of the PVC
If I would have built the fish tank one correctly I wouldnt need to have it run all the time.. The turtles one is much better for consistency but lacks the ability to flow as much water. Basically the fish one creates a lot more vacuum to actually pull water through it once it has started. Which I now believe to be a bad idea in the long run.. I should have just built a dual overflow for that tank.