So tonight I saw one of my black neons up close and it had a raised, irregular white clump on it's side. My pic is pretty bad but it was the best I could get. As far as I can see, the clump looks solid rather than cottony (no movement in the current from it - it looks like a clump of fat would on a roast) I think the skin around it is showing a tinge of reddish color so I will assume the surrounding tissue is also affected. The only medication I have on hand is Kanaplex, and I had enough to do a full dose tonight (it's a 125g tank) and I still have enough left for a half dose - so I be calling the lfs store tomorrow. They keep Kanaplex in stock but sometime they run out & it takes until the end of the week to arrive. We'll see how it goes.
I had columnaris in a 55 a long time ago and this doesn't look anything like that did at the time - I had an acute bout of it in the 55 and 80% of the fish that died showed no cottony symptoms, they just became listless and died within a day.
I do have Blue tetras in the tank and I've seen them nip at the Columbian tetras (hitting them broadside, not just fin nipping) - could this be related to a nip?
I did have something odd in this tank early in the summer - a young Columbian tetra fry I raised in the tank had a white raised patch show up on the caudal fin. It changed position & shape (which made me think of a parasite) and then after about a week I saw where the area must have burst - there was a crater where the what line had been. The fish healed up and was fine. Don't know if this is the same thing with the black neon, but I hope whatever it is, it doesn't trash the tank.
So, would you add some salt too? Don't want to raise the temps until I'm sure it's not columnaris. Catching the fish is not an option - it's heavy with complex driftwood, rocks and plants - at 125g, the tank is too big to catch a tetra!
edit to add; water tested ammonia = 0, nitrites = 0, nitrates = 10 the last time I did a w/c, a week ago.
pic of the black neon today, and a pic of the columbian tetra from the summer;