Honestly, external filters are not really a good option for a saltwater tank. Your biofilter will be your live rock, and other filtration can be provided by a protein skimmer and/or a refugium. Most reefers that start out with an external filter end up getting rid of it, since it tends to harbor and leach nitrates into the water.
If you have a sump, then I really highly recommend you look at the Aqua C Urchin skimmer. If not, there are a variety of hang-on-back skimmers that are fairly decent. Although, since your tank is on the smaller side, you might be able to get away with just doing regular weekly water changes and see if that works for you.