My very simple closed loop system.
1/15 HP pump (3000 gph)
Hayward Sand Filter chamber with modified internals, sinking type beads and nitrate media.
Hayward Cartridge Filter currently with no element.
All plumbing is going over the top of the tank. One suction and one return. Pipe throughout is 1 1/2".
System has been running non-stop with no modifications for several years. Was built for my mahseer growouts then my Brokos, kelberi, xingu, trevally and now dorado.
The pump pulls water out of the tank and sends it to a three way valve. This valve splits the water between the cartridge and sand filter. The valve allows bypass of the cartridge so the pump can remain on while the cartridge element is pulled for cleaning.