I dont completely understand what a return pump is. All i know is that it creates a push for water to go up into the main tank. but i looked to see where i can buy one but no results...i googled it and typed in "return pump" but nothing. is there like a technical term for it? or is like the air pump like for bubble stones?
A return pump is just a water pump, either submersable or external, that pushes water from your sump into your main aquarium. As the water is pushed from the sump into the aquarium, this causes the aquarium to fill up to the point that the excess water gets pushed into the overflow box and then cycles down back into the sump.
Pretty much any kind of water pump can work, but you'll need to figure out what size you need based on the flow capabilities of your overflow system and how high above the sump your tank is.
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The best method is to drill the tank. A 250gph pump, internal or external, should provide sufficient flow; of course, depending on how much flow-through you want and could go larger depending on what else you plan to feed.