If you quarantine them before letting them into your main tank (medicate them if need be) then they can make good additions. You can find eels, barramundi, etc. at the right market.
Sometimes they will not sell them to you live. Its against the law to sell live fish in California from Fish Markets. I have seen baby Tilapia for sell on craigslist. If they do sell them to you I would quarantine the fish and feed it up.