Well, I have some ideas. I would start by checking out reefcleaners.org , I've heard great things about them and they have some cool critters. I would also highly recommend checking out the Divers Den section of Liveaquaria.com. I'll make a list of some neat invertebrates that might fit your tank.
- pitho decorator crab
- staghorn hermit crab (shell is encrusted with briozoans that need filter feeder foods to keep alive. The crab itself is pretty easy)
- arrow crab (might harass small fish and shrimp)
- anemone crab
- anemone shrimp
- saron shrimp (depending on species may pick at coral)
- hairy chiton
- chestnut cowrie
- money cowrie
- globe urchin
- red serpent starfish
- brittle starfish (avoid the green and yellow, they might eat fish)
- flame scallop (difficult to keep, need a lot of filter feeder foods)
- tunicates (same as flame scallop)
- sponges (same as flame scallop)
- blue coral banded shrimp
- gold coral banded shrimp
- orange turbo snails
- zig zag periwinkle snails
- dwarf planaxis snails
- cerith snails (any type, super hardy)
- sea hare (large tank with lots of algae only)
- lettuce sea slug (need the correct food source to live, and a lot of it)
- limpets (should be okay but not 100% sure about SPS coral)
- tiny brittle starfish, chitons, etc.
Aaaaannnndddd that's all I could come up with at the moment. Make sure to research how many of a critter to get and what they are compatible with. If done correctly, you could keep a menagerie of invertebrates together!
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