What size return pump do I need?
Return-pump sizing goes wrong in a predictable way: people buy to the number on the box. That number is measured at zero height. Once the pump is lifting water from a sump up to the display, it delivers considerably less — and it's the delivered figure that has to be right.
What size return pump do I need?
Aim for 4–10× your system volume per hour, delivered, with 5–8×/h as the sweet spot. Below 3×/h the sump stops keeping up with the display; below 2×/h it can't run the system. ReefPicker computes the delivered figure for your specific tank rather than reading the box.
How much flow does head loss actually cost?
Enough to change which pump you buy. Lift is the main term, and plumbing adds more. ReefPicker estimates it from your tank and cabinet height for a sump build, and from tank height alone for an all-in-one — assuming a typical sump lift on an AIO would overstate the loss badly, because there is no cabinet to lift out of.
Can a return pump be too strong?
Above 10×/h is treated as something to dial back, not a conflict — a controllable DC pump throttles down, and buying one size up then tuning it is the usual advice. The real ceiling is your overflow: if the pump can push more than the drain can carry away, the display floods, which is the one direction ReefPicker treats as a hard stop.
DC or AC?
DC if you can. You can tune the flow to the number you actually want instead of restricting the plumbing to get there, which causes microbubbles. It also runs quieter, and the ability to throttle is what makes a slightly oversized pump the safe choice rather than a mistake.