What y'all don't get is head height.in your case it's a smaller pump an head height will be a contributing factor an with this an our low psi pumps size makes difference.one inch pipe will have more water weight an more water to push up an over the tank.so I'd say go smallest as possible in the aquarium world.its one thing ppl don't think about.most one inch drains are blocked to half inch or 3/8 to get a complete syphon.ppl don't realize what a 3/4 inch pipe will flow.it will flow more then ur one inch gravity fed drain.
