unfortunatley your sandsifitng star will most likely die in a 20g. the are big eaters and will quickly eat all the life in your sand. nassarius snails do a great job of stirring the sand bed and cleaning up. a brittle or serpent star would be a good choice. they keep the surface of the sand bed cleaned up pretty good. seahorses are a pretty specialized animal. they like a pretty big tank. a square or tall tank seems to work good. they also like a good amount of macro algae to wraps their tails around and hang out in. a gentle but substantial flow ( more volume than direct streams of water ). like grimlock said, jut about anything will get to food faster than them. they eat pods
, and like live food. very similar to a mandarin dragonette.
as far as what you can have in a 20g that is odd looking...
firefish are kind of odd with their tall spike. bi color blennies kind of look like a horse in the face and are fun. i am not sure about the size requirments, but possum banded wrasses are pretty strange looking. an urchin is also pretty cool.
octopus are very aggressive and escape artisits. they are generally not suggested for the home aquarium.
sorry for the long post
75g RR. 2x250 14k MH
/ 2x54 T5 act. 29g sump. ASM G1X skimmer. phosban reactor. 80lbs LS
, 90lbs LR
, 2x seio M620.
kole tang, bicolor angel, mandarin dragonette, ocellaris clown / BTA , cleaner shrimp, 2x peppermint shrimp, a variety of soft, SPS
, and LPS
corals, orange linckia star.