is to small for a 75 gal
tank. By the time you factor in the space to allow for backflow in the event of power outage you probaby wont be able to run more than 2 gal
of water in the sump and this will end up causing the pump to pull in air if not constantly adding water to replace evaporated. A better size for a sump would be a 20 gal
tall. This would permit you to run the sump with 8-10 gal
of water as well as provide capacity for another 10 gal
of water in the event of a power shortage.
1) Sumps Explained
2) No the tank does not have to be drilled (aka ReefReady). You can purchase hang on overflows from many online venders. I perfer U tube overflows over the C shell style.
3) You would need plumbing from the overflow to the sump and plumbing from the pump back to the tank. Preferablly the pump should be isolate with a true union fitting so you can remove the pump from the plumbing in the event of a pump failure or service. Above the true union you should put a ball valve so you can close off the water before disconnecting the true union. Above this ball valve you might want a one way gate vlave. This is not required but just something some people like to have . This prevents water from flowing back into the sump but permits water to flow from the pump.