After considering lots of options I went really old-school. A canister seemed overkill for a twenty gallon that's going to house a paradise fish (they like gentle current) and a few bottom-dwellers. Plus I forgot to make a gap into the back of the tank stand and don't want to drill a hole in it. I don't want an HOB for this tank- nor another internal filter than hangs off the rim. What I pictured in mind was something like the sponge filter in my betta tank, but larger and to hold more media options.
I am concerned maybe I got a size too small? This one it said on the website "for up to 45 gallon aquariums" and I thought great, that's twice what I need. It wasn't until I had it up and running that found the small print on the box: it turns over 45 gallons per hour. I know there's different opinions on this, but most aquariums should have turnover rate four or five times the volume of the aquarium, if not more. Granted, I don't have huge waste-producing fish, but... the water is still cloudy. My cories look happy, Fabio looks terrible. I tested halfway through the day: ammonia less than 0.25 (barely tint of green), zero nitrite, 30 nitrates. I think that's okay. I'm going to keep a close eye on it and do water changes if needed (although it is nice the hornwort quit shedding so many needles when I stopped dong the daily wc for the fry).