I would like to replace my Zakki sieve with an RDF. I have a 2‘ x 4‘ concrete pad located 19 to 20 inches below pond water level. My main concern is if this placement will work for a gravity-fed set up. My pond is only 2500 gallons so I’m only looking at the smallest RDF models. It looks like the main options are the FREE 20 or the seaside aquatics Eco 20. Right now my zakki seive is fed by a single 3 inch inlet which pulls from my skimmer and bottom drain tee’d together. I am using a 4500 GPH pump on this set up.