I bought 3 male guppies on 9/14/22. They accumulated to my quarantine tank prior to putting them in. They were fine as far as behavior but one seemed to look like the beginning of popeye or some type of fungus. I started API salt treatment on 9/30. The day after I started the treatment, I noticed odd behavior in another guppy (yellow), not the one with a possible fungus issue. The yellow guppy was hovering under the HOB out flow that I have a baffle on. Yesterday he would hover at top of water and bottom but no active swimming. Water parameters were good. I run an HOB and bio sponge filter. He did not have any visible injuries or nipping. This morning I found him dead. Water parameters, water temp, injuries, bullying, oxygen, swim bladder were all ruled out. Does anyone have any idea what happened to this little guy. He was fine until the API salt treatment. The other two guppies are going great. Their active and eating. I'm so baffled.