Who Gives A Fish Forums

I've got a bit of a mystery on my hands. Currently, my tank's stocked with:

9 Blue-green reef chromis
1 algae blenny
1 Coflake Beauty
1 Green coflake goby
1 purple firtideflow
2 perc clowns
1 spotted mandarin
1 royal gramma
2 anthias (1 male, 1 female)

Lately, I've noticed the bigger perc, mandarin, and male anthias have all got chunks missing from their fins. The mandarin and anthias are missing bits from the top of their tail fins, while the perc's lost a chunk from the bottom of its anal fin.

I've been keeping an eye on them, but I just can't seem to pinpoint the culprit - there's hardly any aggression going on. The royal gramma's always had a thing against the perc, but it's never bothered with the other fish.

My gut tells me it might be the Coflake Beauty, but I've never actually seen her attack anyone. I'm stumped. Has anyone else had a similar experience? What are your thoughts on what could be causing this?

I'm guessing the percs might be the culprits here. In my experience, they can be pretty feisty. I had two of them in my 180L tank a few years back, along with two false percs. They were fine with one false perc, but when I added the second, they started picking on it big time. They also got a bit territorial and would often chase my bangaii cardinal and the chromis. I had a coflake beauty in the same tank and never saw it show any aggression, so I'm inclined to think it might not be the one causing the problems.

I'm pretty sure they're true percs, but I've never managed to get a proper count of the dorsal spines. The bigger one does get a bit testy if anything gets too close to its torch, it'll swim at them aggressively, but I've never actually seen it bite. The little one, on the other hand, is tiny and seems to get along with everyone, it even swims with the chromis most of the time. It's definitely possible that one of them is the culprit, but like I said, we're keeping a close eye on them all and might never know for sure unless we catch them in the act.

Nighttime fin nipping's definitely possible, could be our culprit's more active when the lights go out. A pic of the damage would be helpful, might give us a better idea of what we're dealing with. Beauties can get a bit feisty once they're established, but like I said, crabs can cause some nasty damage too, usually more frayed than a clean bite mark.

Chromis size-wise, they're all pretty small, around the same size as the little perc. You're right though, these peaceful shoalers can turn on each other when they're in breeding condition, and paired up they can get pretty aggressive. My percs, both pairs, are definitely a handful, but the damage they cause is usually more torn or split fins rather than clean bites.