I think Hondatek is referring to the Marineland Magnum H.O.T. canister filter system. The HOT means hang on tank, same as HOB
hang on back.
H.O.T. Magnum Compact Hang-On Tank Filter by Marineland - Filters - Fish - PetSmart
On my 20, mind you it's a FW
, I run two Rapids mini canister filters. Each is rated for 20g but we all know manufacturers tend to pad that rating. They come with a rack that hangs on the tank so that they are at least tank level. Kind of like the Magnum above.
Rapids Mini Canister C 80
Like others have stated, I don't think most canisters except these two can be kept above the tank for siphoning reasons.