I was thinking of keeping seahorses, though based on the research I've done, it seems that my tank is a bit too small for some of the hardier species like h. erectus, and the smaller species like h. zosterae are pretty tricky to keep.
I think that I might go for some pipefish instead though, specifically bluestripe pipefish. They're similar to seahorses, but can adapt to a wider range of foods and are hardier. At least that's what I've heard anyways, if anyone's had success with seahorses in a 20 gallon I'd love to hear about your experience