I just bought a large midas blenny and even though the min. tank size is 30 gallons I don't think a fish that grows 5" should be in such a small IMO. They are even known to be more aggressive in smaller tanks, so consider that. It's prob. rated for such a small tank size because they hide, but they swim a lot and only go into a rock crevice for a couple seconds then back out.
Some fish ideas would be purple firefish (great vibrant colors), hectors goby (buddies w/ firefish and cleans sand) and/or yasha goby, some type of flasher wrasse like red tail flasher wrasse. You have to get cleaner shrimp! Put your hand in the tank and it will try to clean your hand. Fire shrimp look great, but they hide b/c they like low lighting. Mine only comes out at night and feeding time, but both are excellent CUC members(eating leftover food). I have a common serpent star (less aggressive than brittle stars) and he always hides too, but it's really cool to watch him eat. Serpent or brittle stars are great if a fish disappears (hides and dies) and you can't get to it. They'll most likely be the first ones to make a meal of the dead fish before it decays and eventually turns into more nitrates. Nassarius snails are also great for the same job and pop up out of the sand at the slightest smell of dead seafood. Never had experience with urchins, but I read they eat algae including coralline algae which many people like the appearance of.
