Hi, I'm in the process of building a smallish (12,500 litre) pond to host some Koi and other fish. The pond will be part below ground (1m-1.2m) and part above ground (70-80cm). I'm installing an aerated bottom drain with a 4" pipe to get the dirty water to the filter units - currently looking at the Oase Screenmatic2 60000 unit (which I know from specs is oversized, but I'm erring on side of caution with manufacturer figures on filtration efficiency). So getting the pipework from drain to filter is all good and gravity flow works. But the clean water coming out the filter I want to then get up a 1.5m rise to the top of a waterfall - just not sure how I achieve this? Clearly I need a pump after the filtration unit, but how do I ensure correct flow rates such that pump does not run dry (if piped directly to filtration unit outlet) or do I need some sort of connecting pump chamber such that as the pump pushes water out and up to waterfall, gravity ensures that water rate incoming from bottom drain maintains the water level in the pump chamber? Or am I missing something and/or overcomplicating things!!