Filter recommendation on 20 gal. FOWLR

If you have the skimmer and live rock, you won't need a lot of other filtration. Maybe use one of the HOB filters for circulation, maybe puts some floss in it if you want a little mechanical filtration. Just clean the floss religiously to keep the debris from breaking down and feeding algae.

IMO, keep the water changes up and provide the tank with a good clean up crew it will be fine with just the skimmer and LR. put in about 1/3 of the tank as LR, coral sand, seeded with live sand. DONT overstock and alsong as you are using quality water, e.g. RO/Di water then you will have no problems. Just to give you an idea, i have seen a 20 gal set up with a cannister filter at 1800 lph using bleached coral as the media, and running a overflow / surface skimmer connected to the cannister. the tank was using coral sand seeded, 1/3 of the tank was LR, the inhabitants were 1 barbershop shrimp, lots of hermits, a few turbo's, 2 ocellaris clowns, 1 banded pipefish and it was a very healthy system. for corals it was had 1 anemone, few zoonathid's, and some various other corals, using T5 lighting and the tank was perfectly clean. Another tip is try to get lots of coralline algae growing to keep the nitrites and nitrates down aswell as phosphates. i am not saying this is the system you should run, but i am trying to give you an idea of how similar systems can be maintained.