Account for the whole circuit
Head height and friction through pipe walls, bends, valves, diffusers and media consume pump capacity. Air pockets, kinked hose and a clogged prefilter add avoidable losses. The published rating cannot know the geometry of your cabinet.
Return design changes what the same flow achieves. A narrow jet may discomfort fish yet miss sheltered debris, whereas a spray bar or divided returns can distribute momentum more usefully.
Measure without damaging equipment
Divert the outlet into a graduated container for 10–20 seconds while ensuring the pump cannot run dry. Divide litres by seconds and multiply by 3600. Repeat and average, because timing a short sample creates error.
Record clean and pre-service results. A sudden decline calls for checking obstructions, trapped air and hoses; a steady decline may define a sensible mechanical-cleaning trigger. Never diagnose biological media from flow alone.