Help sick fish i think

[Superdod]

Hi looking for some help/ advice some thing is destroying my maroon clown and one of my velvet jewels fins and tail ! This has just started since I introduced a powder blue ? I have a couple of velvets 1 clown 1 regal tang 1 powder blue tang , 1 brown tang and a yellow butterfly , also 1 hermit crab and a decorator crab !!
The velvet looks as if it is on its last legs , the clown seems busy enough but has started swimming against the current from one of my power heads ( he never done this before ) my water parameters are good ! Any help or advice would be great full

 

[Reef79]

I'm pretty. Sure decorator crabs aren't reef safe

 

[Reef79]

May be getting ahold of you're fish at night just a thought

 

[Superdod]

Lost the clown fish :-( think it was fin rot treated my tank no other fish seem to be affected !!
Thanks for the reply

 

[catmel]

Could have been.. might have bene a nip from a crab or another fish? I know high ammonia can cause fin deterioration among other things.. did you check your water parameters?