Panda cory with no tail

I have 3 panda cories in a 10 gallon quarantine tank with 5 juvenile neon tetras. All were bought on the 9th, 4 days ago. I noticed one of the cories no longer has a tail. The fins are gone just leaving a stump.

The tank is cycled, no ammonia, heated to 78. The rest of the fish are behaving and eating normally. Could the cory have been attacked by the tetras? I medicated the tank 2 days ago with antibiotic and antifungal, and I've never seen fin rot progress that fast even on the worst betta.
Also, should I euthanize or try to get it to recover?

 

I kept neons with corys and never once saw them even get near each other let alone any aggression. I highly doubt it was the neons. Is it possible it could have gotten stuck to the filter or behind the heater? Its also definitely not fin rot. I have had fin rot on corys and it will not progress nearly that fast.

Could you post a photo? It might help with figuring out wither to treat or euthanize.

 

What ever caused the Cory Cat to lose its tail likely originated in the sellers tank, not yours.
Might try to return the Cory for an exchange or refund. The tail won't grow back.

 

What ever caused the Cory Cat to lose its tail likely originated in the sellers tank, not yours.
Might try to return the Cory for an exchange or refund. The tail won't grow back.

This may be true. However for it to disappear over night? It sounds more like injury to me then disease.

 

I'm not sure if it disappeared overnight per se. I hadn't taken a very close look at the fish until today (they're in a tote bin so it can be hard to notice things from a top-down view), so while 4 days isn't long, it's not instantaneous either... In any case, I have no idea how rapidly this progressed over those 4 days. I only wonder if it was the tetras because they can be nippy with eachother.

the tank uses a sponge filter. The heater lies on its side on the bottom of the tank, but there's nothing about it that would let a fish get stuck on it, let alone rip off its fins.

Can't post a pic right now, but it looks an awful lot like this: