The other day I noticed one of my fish either sitting at the bottom or top. It was just the one. Now two days later it’s dead and one more fish is bobbing at the top still alive. None of the other fish are doing that tho. Doing a 50% water change right now. Water perimeters before water change where 10ppm nitrate 0 nitrite 150 hardness, 80-120 kh, 7.6 ph, 0 chlorine. The first fish that Dora’s was a balloon Mollie and the one floating now is a cardinal tetra. Tetra is being very lethargic barely responding to anything. Any help for possible diagnosis or solutions would be appreciated also water temp 80f.