r/glofish

Purple tetra at the center of the video

Six in the tank (with 2 snails and shrimps). He is not interested to eat while the others go at it as if they did not eat for a week. He is slower than the others. I thought the top fin or back fin may have issues as they are not as straight as the others but I don’t think there is anything there. Got them 2 weeks ago. Tank fully cycled, 0 ammonia, nitrite or nitrate, kh 4, gh 9 and ph 7.5

Lost a fish a few days ago, gasping, dosed with Mt labs Gill and fluke 7 days ago. Have done a partial water change. Parameters are spot on. Have had these fir 4 months and now have a mystery illness. Can you help?