I use RO water and seachem products to get the right gh/kh/ph because of how hard my water is (limestone aquifer source). My pH is 6.5, tank's been running about 6 months. Ammonia and nitrites are 0, nitrates 20.
I have 6 black skirt tetra, and had 14 cardinal tetra. Lost 8 of the cardinals in the last two days.
I introduced 4 about a week ago, so I'm pretty sure something was up with them.
They all die of the same thing: skin along their blue stripe somewhere from the dorsal fin to the tail begins to grey symmetrically on both sides (left and right). This grey progresses until they have difficulty maintaining buoyancy and start mostly trying to remain vertical (face in the gravel). Eventually they end up floating at the top or stuck to the filter intake.
One guy had this from his dorsal all the way to the tip of his tail, basically half his body, and when I pulled him out, his tail fin was all but gone.
Please help. I tried to take pictures, but they did not show up well with the light and the faint greyness/color washout. Everything I'm reading says neon tetra disease, but I'm also reading cardinal tetra are supposedly one of the few species immune.