there are some species of starfish that will be good in your tank. seahorses are out, they need to be in a tank by themselves and without LR
because they are easily killed by other fish and by apistasia and other hangers-on on LR
. also, they need to be target fed, and this is more easily accomplished in a smaller tank. there are good and bad things said about urchins and cucumbers. i personally like cucumbers and think they do an excellent job stirring up the sand and eonsuming detritus and whatnot.
jellyfish are a no for most tanks, they need a lot of vertical swimming space and will consume other inhabitants in your tank.
shrimp are excellent, they are amusing to watch and also help keep the sand bed clean. some good kinds are cleaner shrimp (skunk and banded), camel, etc. the "cleanup crew" should also include a good number of hermit crabs (i like blue legs, cheap and do a good job) and a lot of snails.