Flakes are baaad in saltwater. And frozen food is usually fortified too. Avoid flakes and pellets, they contain ash which turns into phosphates in your tank = ALGAE!
If you have a GOOD skimmer, you won't need a filter. There are a lot of pieces of crap out there that claim to be skimmers. Avoid Sea Clones. You can get a good skimmer for less than $100, people seem to like the Coral Life Super Skimmers a lot. Any other good skimmers are going to run you in the several hundreds of $$$ range.