I recently cycled my 20 gallon long tank and I put in 3 albino cories, 6 rasboras, and my betta Comet. Soon afterwards, my rasboras started dying but the other fish seemed fine.
I went on a 5 day vacation over Christmas and when I got back, Comet had terrible fin rot. He had just gotten over severe fin rot before and his HM tail had grown back almost completely. Now he looks worse than and I have no idea what caused it.
I tested the water with the API test kit and here's what I got:
PH: 8.2 ppm
Hardness: 180ppm (highest my kit went up to so it could be higher)
Everything seemed normal so what could be causing my betta's terrible fin rot? (and the death of my rasboras) Comet is in bottled spring water with 2tsp salt per gallon and stress coat right now for treatment. Here's some pics so you can see what happened to his tail.