Using a canister filter on a fish only salt water set-up
 

posting for a friend: He wants a small tank (prob no bigger than a 40 gallon), and wanting to know if a larger capacity canister filter is ok for filtering or is a skimmer still mandatory even tho it is a fish only environment?

Skimmer is better as mechanical filters don;t really remove dissolved organics.
But as long as he's good with his water changes, a skimmer is not make or break...even for a reef tank.

I have successfully kept numerous tanks running on a canister AND a skimmer. Works just fine. I currently have a 39 gallon bowfront running on a new Fluval 406 with carbon, phosban, and 2 media trays with live rock rubble. Tank has flourished. Also running a Hydor slim skim on it.

That being said, just like gobytron mentioned, as long as the user is diligent enough with regular water changes, a skimmer isn't manditory. If he is a beginner, I would recommend a skimmer.