Something is nipping my fishes tails

[Madreefer]

I'm betting on the tomato clown. They get very territorial if another fish comes around their "spot" such as an anemone. At least thats what i've seen in my old tanks.

[RuGlu6]

Are you sure your LR has no pests?
Yo might have bobbit worm aka Eunice worm. The stay small for a while and after a while just explode in size.
The only way to be sure that you don't have one is when you started with Dry Rock not Live Rock.
If you did start you tank tank with dry rock the you can bet on the clown.
But you don't see anything during the day and that can be something you don't know about that hiding in you tank's dark corner.