The typical 1″ bulkhead can handle around 600 gph of draining water flow. This doesn’t mean your return pump should have an output of 600 gph or less. Pumps lose the amount of water they can push the higher the output nozzle is. This is called head height. As the height increase, the speed at which the water can flow decreases dramatically for most pumps. You could actually have a pump rated for a little more than what your drain can handle and not have to worry about your aquarium overflowing. Be careful when doing this though. Use valves to throttle back the pump and keep the drain line clear of debris that could impede its ability to drain.