Help diano tail injury

[hmead]

Tank size: 10g
pH: 7.4
tank temp: 76
Water parameters are fine.

Tail injury! Please see pic.
He is isolated in a breeder tank set up now. Sulks near bottom. Swims only when absolutely necessary (as you can imagine) & tries to hide. Hadn't seen him today, so i went looking. Seemed fine yesterday night. Haven't seen him eat today.

Volume and Frequency of water changes: %25 every wk

Chemical Additives or Media in your tank: carbon filter, prime and stability w each water change

Tank inhabitants:
2cory cats (had for 1yr)
1 female Molly (recent addition- 2 wks)
2 (m/f) platty (had for 1yr)
2 oto (had for 2yrs)
2 neon tetra
(had for 3+yrs started w/ 4)

1zebra diano
(had for 3+ yrs started w 4. Two jumped tank (old smaller 5gal one) one culled as procaution b/c of curved back&abdominal growth.

The Molly had a mate. Lost him a few days after the addition & treated whole tank with lifeguard all-in-one. No signs of sickness or trama on male Molly's body. This was a week or 10d ago. All the rest of the fish seem fine.

Could I have a homicidal Molly?!
Should I euthanize? Treat? Wait it out?

 

[FroFro]

Is the red part of his fin swollen or oddly shaped? Like is it bent at an unnatural angle? It could be a severe bacterial infection if the red spot is only on the surface of his skin. It's hard to tell from the photo as I'm not an expert

 

[flchamp89]

Is it just the picture or does he have some red around gills as well? You say ammonia is fine?

Sent from my SAMSUNG-SM-G890A using Tapatalk

 

[NickAu]

Should I euthanize? Treat? Wait it out?

I have no idea what is wrong.

What I would do is remove the Danio from the tank, Put it in a hospital tank, then wait for advice from somebody who knows what it is and how to treat it, If its say an injury it should start getting better if its some bacterial thing it may save your other fish getting it, In both cases fish are easier to observe and treat in a hospital tank. Some treatments may harm the other fish plants or even the good bacteria in your main tank.