Tail rot or torn fin?

[impossible]

Saturday afternoon I added three pygmy gouramis to my 20 gallon tank (previously occupied by two bloodfin tetras for a couple of months). They seemed to acclimate with ease and settle in fine, and they were far less shy than I had been led to expect. Sunday evening I noticed that one fish appeared to be missing a section of its tail fin. There's a black mark at the point where the tail fin abruptly vanishes (which you can see in the attached photograph) and possibly a very faint black line along the top edge of the remaining part of the tail. I hadn't noticed this earlier, but I also wasn't familiar enough with gourami tail shapes then.

Is this definitely bacterial fin rot, or might it also be (for instance) that one of the other fish has attacked it? The gouramis have been croaking, which I understand to mean that the males are verbally sparring over territory or mates. (I did request two females, but pygmy gouramis are apparently very hard to sex.)

How long would fin rot have to go on for that much of the tail to be gone?

Any advice on how to handle this would be very much appreciated.


Additional information:
The tank is at 78 degrees with a pH of 7 and ammonia, nitrites, and nitrates of 0ppm. I did a couple of ~25% water changes last week and a 15% change on Sunday.

I did add some new PVC pipes and new plants recently, including a large background plant that hangs from the aquarium rim, with a large plastic base that fish sometimes hide under.

 

[Saltypuppy]

Fin rot is known to have black edges and it can cause chunks of fin to just disappear. Is the edge ragged? Also the best thing you can do, (Since it apppears to be minor) Is to change the water more frequently. Aquarium salt might help too.

 

[impossible]

Fin rot is known to have black edges and it can cause chunks of fin to just disappear. Is the edge ragged?

Yes, looking closely I see that the edge does look ragged. I guess this points to fin rot. These fish were shipped cross-country, and then an experienced fishkeeper put them up in a bucket for a week. The water quality and food quality were good, but I think the lack of hiding places might have stressed the fish...

Also the best thing you can do, (Since it apppears to be minor) Is to change the water more frequently. Aquarium salt might help too.

Thanks for the advice. I'll add aquarium salt and change the water daily.

I'm also wondering whether I should put this fish in a 10 gallon quarantine tank (assuming it has finally finished cycling). Right now, I'm inclined to leave it in the main tank, as it is swimming around and interacting with other fish some of the time.

 

[Saltypuppy]

I would seperate and do it that way. If the others are healthy there is no point in medicating them. Salt is a medicine...

 

[impossible]

I would seperate and do it that way. If the others are healthy there is no point in medicating them. Salt is a medicine...

Thanks. I wound up quarantining him a few days ago. He's on salt and Maracyn 2. The Maracyn 2 is just kicking in.

I think what I discovered is that while the water in my main tanks tests well on the usual parameters (hardnesses, pH, nitrites, nitrates), the problem was that my gravel was not being sufficiently cleaned, so there was a lot of junk down there which would occasionally get kicked up when I cleaned. I think I almost have it under control, so if I can save this fish, everyone should be fine.

 

[Saltypuppy]

Thanks. I wound up quarantining him a few days ago. He's on salt and Maracyn 2. The Maracyn 2 is just kicking in.

I think what I discovered is that while the water in my main tanks tests well on the usual parameters (hardnesses, pH, nitrites, nitrates), the problem was that my gravel was not being sufficiently cleaned, so there was a lot of junk down there which would occasionally get kicked up when I cleaned. I think I almost have it under control, so if I can save this fish, everyone should be fine.

That is great to hear. Sounds like you are doing a good treatment too.