I had 6 healthy Bloodfin Tetras in my 20 gallon tank (with my Cory cat and Dwarf Gourami). They have been active and happy for many months. Four days ago during my regular water change/maintenance, I reversed my tank landscaping. That is, if a plant was on the left, I moved it to the exact spot on the right. Everything inside the tank was just reversed so it's a mirror image. Soon after I noticed that one Tetra wasn't as active and didn't eat as much any more. He still ate and moved around just not as much. Earlier tonight he didn't look any worse but later on he was belly up and a total goner. It was very fast. So, my question is - does radically altering the landscaping potentially cause a lot of stress in weak individuals that could result in death? Or was this just a case of bad timing? My water parameters are perfect so I'm not sure why this fish would just die.