Who is the culprit?

In one of my 75g temp holding tanks I am having issues with fish showing of with big hunks out of their tails, like some underwater moth has been chewing on them, but I can't figure out who is doing it because all the interaction I see are pretty peaceful.

Here are the inhabitants:
7 Spilurus cichlid growouts @1"
7 Dollar and Central Longear sunfish growouts @1"
1 adult rainbow cichlid
2 female bettas
3 Pearl gouramis and 2 Gold gouramis
7 various botia loaches (yoyo, historanica, striata)
2 Clown loaches
1 Redtail loach
1 Circumcintus eel
1 neon goby
5 Kuhli loaches
5 Mystus bimactulatus
2 cherry barbs
1 tiny Oscar @ 1.5" (now deceased)

So the weird part is that only the top/middle swimming fish are showing up like this. The kuhlis et al are fine. The fish that seem to be getting the worst of it are the sunfish, spilurus, bettas and the Oscar. Today I found the Oscar dead. But I have't lost any others this way yet. My first instinct would be to say it is the cichlids and sunfish, but I don't think it us because they were previously in a tank 1/4 the size together and had no ripped fins at all. When I added them to this tank they immediately started showing up like that. I added the baby Oscar recently to let it grow out before moving up to a tank with bigger fish and the first night it had it's fins shredded. Just found it dead about a week later, and missing all it's fins.
My second guess would maybe be the loaches, but, except for the Clowns, I have had all of them for a couple years with all types of fish and never had a single issue. So of the fish left what do you think is doing it? My suspicion is the Mystus Bimaculatus, but everything I have read about them said they are very peaceful!
What do you guys think?

I should have added that the gold gouramis were added AFTER this started happening. Its not the cherry barbs for sure, they also are quite old (hence I only have two left) and have been in many tanks without causing a problem. I did wonder about the clown loaches, but are they really that nippy? Whatever this is is eating more fins than the skunk loaches I once had, and those are known for being quite nippy.

But this is way worse than my skunks ever were, hence why I don't think the loaches... So no one knows whether Mystus Bimaculatus can get nippy?

To be clear there are no skunk loaches in this tank, only clowns and some smaller botia species. I was just saying for comparison that this nipping is worse than the skunks I had and I don't think clown loaches are normally worse than skunk loaches. The water is fine, that is not why the Oscar died - there are much sensitive fish than Oscars in there, and of the 40 fish in the tank only 5 are over 2" and I bet almost half are at only 1". Definitely not over stocked for a 75g, though it would be if everything was full grown. It is mostly just a growout/holding tank.