Your tank is a bit small. Why not just put some Rowaphos in a small filter bag and hang it in your tank. Keep it in an area of flow and it should be fine. Or if you use a canister filter you can put the mesh bag in there.
Hanging it in your tank won't do the trick. On my 33g I put it in a bag in an AquaClear 20 ($18) that I have turned way down. It works fairly well.