Something is nipping my fishes tails

[phar]

So I've noticed that a few of my fish are missing parts of their tails. In the last week I'll go to bed and wake up and a little nip is gone.
During the day, there is no fighting and they all seem to get along. The only thing I could think of is a really aggressive female tomato clown. (I have a pair.)
My live rock has no pests. I have one medium hermit crab. (No bigger than a quarter at the most.) Could the crab be nipping their tails at night?
Any ideas as to what it could be?
Here's the fish I have in my tank:
Small Bariene Tang (Nipped tail this morning...One of his streamers is missing)
Small Yellow Tang
Small Multicolour Angel (Badly nipped tail.)
Pair of tomato clowns
3 green chromis
lawn mower
Various LPS and SPS.
Thanks,
Patrick