hi all im working out an idea for my 120 gallon turtle tank that i will be rebuilding i a few months. being as though it is a turtle and African cichlid tank i have a lot of waste to deal with. also i have a layer of 1 inch gravel on the bottom of the tank (it is big so that the turtle cant eat it) so it is hard to gravel vac. so my plan is to set up an under-gravel current that will sweep the waste into the filter so that it can be removed. so i already know how to get the water to go to the bottom of the tank without a risk of overflowing the tank. the part i am working on right now is the tank overflow. so to get my idea to work i need to be able to draw water from the bottom of the tank. so my plan is to have a bulkhead at the bottom of the tank that will open up to under the gravel. this will then take a 90* bend to a Tee that is at the water level and is open at the top. this up pipe will have a ball valve on it to regulate the flow through the pipe. at the water level there will be another bulkhead that will be a surface skimmer and i will use the ball valve to regulate how much water travels from the bottom of the tank so that i can utilize the surface skimmer. this will then take another 90* bend to drop to my sump. i am also planning on installing an emergency overflow bulkhead that is slightly above the waterline so that if something goes wrong i wont have a mess on my hands my sump return line will be split to go to the top and bottom of the tank. this part is already a proven design that has been running for almost two years with out issues. what do you think of my idea? this is just a ruff sketch that still needs to be refined but you get the idea. 
