If you keep a clown goby with anything that might nip, you'll see that they taste bad. I used to have one, and my blue damsel instantly started pestering it- hitting it with his tail and such. It didn't budge. The damsel took one nip, froze, zipped across the tank, and sat in the corner looking unhappy and mouthing furiously. He did NOT like how the goby tasted. It's apparently the slime coat, as he didn't do any damage to the goby.