A few days ago, I got 4 neon tetras at petco, and after testing the pH and ammonia levels too much, I put the fish in. After 48 hours, one of the neon tetras started to float, and just wasn't as active as it was before, and sort of just floated in the filter. The color of a neon tetra was not there anymore, and it almost seemed like it was bitten. It was barely moving, so then I quarantined it and it would not eat, and later stopped moving. Also, it seems like the 2nd fish went missing, and I have no idea where it is. The ammonia level was at 0, and the ph level at 6.6 in a 12 gallon tank. There are still 2 fish that seem to be completely fine, showing no signs of bad health. I was planning on adding a few more neon tetras, and solving this problem, and I feel quite desperate. Thanks in advance!