I would put a standpipe in the hole so the top is at about the halfway hight of the tank. Then use that as the inlet to a canister filter. This would make it very easy to do 50% water changes, using a couple of cheap ball valves, one to direct the filter outlet to a drain, and one to allow tap water into the filter outlet line. Priming the filter would be extremely easy, since no siphon starting is needed. At worst the water line in the filter circuit would be at the tank water line, so the filter just has to push a little air over the top of the tank to get started.