I uses a 25 gal
rubbermaid that is only half full. I uses a maxijet 1200 to pump water from the sump to the fuge and I drilled a hole in the rubbermaid to spill back into the sump.
A 5 gal
would be fine as long as you tank is not huge. Might not give you alot of benefit if your tank is big other than a haven for pods
*180 gal Display, 100 gal basement sump, 33 gal refugium, 3x250 MH, 2x160 VHO actinics, zoos, some softies, LPS & lots of acros and other SPS.
*100 gal prop tank plumbed into main system w/ 2x96 PC lights and 1x150 MH,