p/s as nicky have said is protein skimmer....
water to go down the tank, overflow system... usually custom made tanks comes with a built in overflow box with a drain at the bottom... in my case, i am using a prefilter overflow box....
it's true, if ur return pump fails water will overflow the sump... solution:
1) maintain the water in such a way that even if REVERSE FLOW happens, the water in the overflow box will dry up first before it can fill up the sump to overflow level....
2) make ur return pipe nearest possible to the water surface so once it grasps air, the REVERSE FLOW will stop... another way is drill a small hole at the return pipe just right above the water surface, it will weaken the return pressure abit but in case of return pump failure, it will cut off the siphon due to air sipping in the whole.