sick neon tetras

Hello all, this is my first post here so I'm hoping somebody can help me! My tank is relatively new - 6 months roughly. it is a planted tank and has only just started establishing, but t was fully cycled before i added any fish. On bad advice, i started my collection with a group of neon...

I have a 10 gallon community tank, with one betta, six neon tetras, and two albino Cory catfish. I’ve had this tank for a little over a year, and do 25% water changes once a week. All the fish get along very well, and up until two months ago, have been very healthy. About two months ago, I...