I hope to high heavens it isn't what I'm dealing with, not very well I may add.
This is a bit lengthy, but last year, I bought some cheap neon tetras, which were skinny and had faint black markings on the top of their dorsals.
I thought these were due to shipping stress and ammonia burns, which I've heard can cause blackening.
As I wanted to build them up a bit prior to putting them in a tetra tank which had had some mycobacterial infected fish, (something healthy fish aren't susceptible to catching, but which can decimate stressed fish,) I put them in a 25 gallon tank with only a few other fish present.
I bought some Harlequins from the same place, which had a number of ailments, all of which apparently cleared with various treatments, apart from what appeared to be a thick, black coating on the leading ray of their ventral fin, and some black on other fins.
This initially spread to plants (and killed an entire, enormous Water Wisteria, among others) when they rubbed their bellies on leaves in breeding behaviour.
This seemed like a fungus to me, something I feel has been amply confirmed in the interrim.
The Harleys were quarantined in a 15 gal. Walstad intended both for them and a betta in an adjoining 10 gallon, who had always been perfectly healthy, and who was have an upgrading.
Some time after this, I bought two stunted-appearing Crowntail bettas, also from the same place, with health issues, the male having had a number of his very long fin rays eaten to stubs, (although he was recovering when purchased, and is the only one to do so) and the female (sold as a male wild-type, claimed as possibly an Imbellis, due to radical-looking stress striping) missing virtually all of her extensions, part of her lumpy black ventral fin near her vent, and being unrecognizable (except to me, although I thought she was some cross) as a Crowntail.
She was also (unbelievably) unrecognised as a sick fish, as was also the male.
All of the fish very gradually increased in beauty and health, and have fabulous colouration - it all seems so surreal.
The black fin issue, apparently some type of fungus, has been treated with a number of meds. (starting with the Harley's) over about, I suppose a 6 month period, (long nightmare, not sure of time-span but at least that, I should think) including several wide-range antibiotics, every antifungal treatment available, Metro plus and combinations of everything I could obtain to throw at it.
The betta in the adjoining tank was evidently, despite care taken, splashed during the frequent waterchanges required on the Harley's and developed a voracious black tail rot which not only consumed the top half of his caudal fin and spread across the entire lower area of his tail, now blackened and charred in appearance, but grew out in large and increasing lumps on his side and topline.
Like the others, his colours are still beautiful, he still eats eagerly and swims with remarkable ease, considering, although he spends more and more time lying on the ground, something he never did previously, generally looking out at me, as he knows 'Mum' should be able to fix such things.
I've tried everything - there's nothing left.
He, the beautiful little guppy in his 10 gal. and the Harley's will all have to be put down with Clove Oil, plants chucked, anything salvageable sterilized with both bleach and rubbing alcohol.
And earlier today, when feeding the 25 gal. containing the affected Neons, I noticed my self-collected Central Mud Minnow, who I've had several years, has suddenly developed a large, dark lump on his side.
Not only is the oto which was one of the first two fish I bought when I last got back into fishkeeping in that tank, with other otos, but I've only just added 2 big, beautiful, healthy new ones - condemning them to death as well.
Assume the black, fungus-like lumps are highly contagious.
Assume any fin blackening, however faint, may be some version of the same thing.
A number of horrible ailments have been spread from the giant fish farms now supplying a large percentage of pet shop fish, although a great deal of Internet searching brought me no specific info of any kind regarding this.
Luckily, I always wash several times, (up to the elbows, as well,) after finishing in one tank, before touching anything else fish-related, and keep my syphon hose in a bucket of bleach solution between tanks and just rinse the heck out it every time, so I hope my other fish/tanks aren't affected, and I've seen no signs as yet of this elsewhere.
If you have not been taking precautions with any other tanks you may have, and even if you have, I'd suggest keeping a careful watch out.
I should have put down the affected fish some time ago, although I can't bear to - but I can't leave them to be eaten alive by this, and the longer I leave them, the higher the chance of further spread and the loss of more pet's lives.
But if you do come up with any potential treatment which might be successful, please let us know.
I'm very much afraid that this will be far more common than currently known.
Edit: typos
thanks in advance
