I got a diamond goby 3 weeks ago and I haven't seen it for 4 days now. I haven't found any fish on the floor. There are not too many places in my tank I can see. This happened one other time a few months ago in another tank. Never found anything. but there was a black crab hitchhiker that got very big and I suspect the crab. But I know for a fact that all there is in this tank is two clowns, a cardinal, 6 line wrassie, fire fish a cleaner shrimp and a peppermint shrimp.