[KarlBob]

I think it's still too early for a sand-sifting starfish. It will eat everything it can find in the sand bed, then starve to death. The heavy predation will reduce the effectiveness of your sand bed, then the corpse will produce a bunch of nitrates. Give the sand bed some more time before you add any "sand sifters", whether starfish, sea cucumbers, gobies, or blennies*.

For churning the sand, I recommend nassarius snails. They rest in the sand but come out to find food, so they're not dependent on the inhabitants of the sad bed for their diet.

Pistol shrimp and goby pairs also keep the sand stirred up, but some people get upset when they start bulldozing the tank. Rock piles can be disturbed if they're resting on the sand instead of the glass. Small frags may occasionally wind up deep inside a tunnel, propping up the wall, especially if they were sitting on the sand bed. Even snails are not immune. Not only do their shells make good tunnel supports, but the meat inside is tasty, too. Despite all that, pistol/goby pairs are still great fun to watch.

What kind of blenny do you have? I'm no expert, but I believe many blennies and gobies can coexist. They look similar to us, but I don't know if they look similar enough to each other to stir up the kind of aggression that two gobies or two blennies would cause.

* Note - This specifically refers to "sand sifting blennies"/"sand sifting gobies". Many Gobies and blennies do not rely on sand bed critters for food, and don't require the same waiting time.