15 Gallon Tank Investigating Mysterious Tail disappearance - Need your help

Alright. I have a 15 gallon tank of Neon Tetras. (and a small Pleco)
There was 16 of them Tetras. And I have this tank for 4months, well cycled, never had problems with the bio filters.
I change water once or twice weekly, usually around 10%-15% each, and it is mildly Salted at 3ppt. (roughly 1 tbspn of NaCl per 2 gallons)
I have an API test kit now, and just today after another mysterious death I did a test:

Ammonia/nitrite at 0, nitrate at 10. Hardness of the water is around 80, pH is around neutral, and at 80'F.

you might be thinking why is it salted?
This is cause a mysterious situation had occured since around 2mths ago, and I am clueless.

1 by 1, I will occasionally see my Tetra losing their tails, and with it, usually a swimming problem. (which I contribute to the tail, but it really looks like it can be swim bladder related too the way the fish can't even float 'upright' in shallow water.)

Today I had another one, and I took a photo of Bob. (yes, his name, is Robert Paulson..)



Notice the tail is completely gone. This is a common situation in all the tetra found dying in this tank, no exception.

My question here is... is this caused by fin nipping by other tetras???

This dead tetra here is almost 2cm, and is one of the 'bigger boys' in the tank.... I am surprised he just died like that.

and he is not alone, 4 months ago, I had 20 tetras when they were all less than 1cm long, and now only 15 remaining.

Can this be a fin rot disease that's plaguing them slowly? I want to know if I need to start using the right medications, opinions very welcomed, please help me solve this.

 

Could just be them nipping at each other, although I'm not quite sure neons are known to nip.

 

If the picture is fresh, i.e., the fish is not yet decomposed, the whitening of the red section suggests neon tetra disease. I don't know if there is an effective treatment.
Otherwise, some sort of aggression. What type of pleco do you have? Catfish do mysterious things after dark.

 

Could also be saddleback/columnaris. I did not have success treating a guppy that had it. It will continue taking the tail on up the back of the fish, eventually causing swimming and floating issues.

 

If the picture is fresh, i.e., the fish is not yet decomposed, the whitening of the red section suggests neon tetra disease. I don't know if there is an effective treatment.
Otherwise, some sort of aggression. What type of pleco do you have? Catfish do mysterious things after dark.

I have a small bristlenose, he is around 2cm long, but I have never witness him interacting with others... I be alarmed if he is the cultprit!

Could just be them nipping at each other, although I'm not quite sure neons are known to nip.

Originally I thought so too, though this dead neon is 'one of the largest tetra' in the tank... to see him go down 'overnight' like that is ... concerning. So I doubt that's the case... though can never rule it out entirely...

that.. and that it is 'recurring'...

Could also be saddleback/columnaris. I did not have success treating a guppy that had it. It will continue taking the tail on up the back of the fish, eventually causing swimming and floating issues.

I have assumed that can be the case too... and guess what, this dead happened during a Furan2 treatment for bacteria infections... so it might not be the case.