Fin Nip Or Fin Rot?

[Discus]

I have just bought 5 cardinal tetras and one appears to have fin rot, it is straight across the rear fin. I questioned what it is because it was in a tank with lots of other species and it doesn't have the tell tale bubbles.

Thanks

Oliver

p.s. will take some pics asap

 

[Yazan]

What size tank?
How long has it been setup?
What are water status (ammonia, nitrite & nitrate, Ph)
What other fish are in the tank?