If you are using live rock that will be your biological filtration. Is your tank drilled for a sump? I prefer to run a sump on tanks that are bigger than "nano" tanks. You can hide your protein skimmer (as well as other equipment) in your sump and add a refugium if you would like. It will also give you more water volume. Canister filters are not very good in saltwater systems but some people use them to put carbon or some other type media.