You'd be a lot better off buying a canister filter for a 30 gallon tank than trying to build one. It would be less than $50.00 for a canister that can handle a 30 gallon tank. The pump...media....hoses and parts would cost you twice that with no warrantee.
i agree with dasarab.. a fx5 would blow those poor fish against the opposite wall! haha.. look for a canister that is rated for a 30 gal tank.. like 30-50 gals.. would be great. i use fluvals so i cant help much when it comes to other canister brands..