Tetra disease and treatment

[goose70]

I seem to have a slowly but steadily progressing "wasting" disease of some sort making its way through the tetra population of my blackwater aquarium. The fish develop dark blotches on their scales, lose color and generally look like someone rubbed their sides with sandpaper. It seem to impact only one or two fish at a time, but they do not recover. It does not appear to be impacting non-tetras (Angles, Plecos) or -- so far -- the cardinals. Three weeks ago, I tried a broad spectrum antibiotic (can’t recall which one), but it did not seem to help, or at best, it slowed the progression.

Everything else seems normal. The pH hangs around 6.5-6.7, amonia is clear when I test, and I do a 20% water change every other week. The lights are over a year old and need replacing, and the plants are no longer growing as robustly as they were, but I can’t pinpoint any other issue.

Any suggestions about what may be causing this and a course of action?

Thanks much.

 

[Delapool]

Hi, I would have gone for a bacterial infection as well. Has temperature increased in the tank? Are they still eating and schooling well if affected?