My clowns and anemone crab are absolutely fine together and have been from day one. The anemone that the crab lives in is about 3-4" when open. Generally the anemone crab will stay towards the base of the anemone, while the clowns what to swim in through it's tentacles. If the clowns leave the anemone, the crab will come up into it, but not for long. Every once in a while the crab gets into the anemone and throws up his claws to chase the fish away, he doesn't have much luck, they know he can't hurt them. I have two GBTA, one RBTA and a mini carpet anemone in my 155 and the clowns bounce between all the anemones, the crab stays in the one GBTA.