Less than a week ago, I stocked my finally-cycled, lightly planted 29 gallon with 6 neon tetras and 4 rummynose tetras. When we were adding them, my son dumped in a piece of dead fish that was floating in the bag with the tetras... I wasn't happy about that, but it happened. They settled in very quickly, and followed this pattern: Day 1: all tetras schooling together Day 2: separated out into two shoals Day 3+: occasional loose schooling, but mostly each fish happily doing his/her own thing On day 4, I noticed only 3 rummynoses zooming around. On close inspection, I found the missing fish, who was quietly hovering about an inch from the bottom of the tank behind a decoration. I didn't take much note at the time, but yesterday evening (Day 5) I realized that this same fish was doing the same thing. When I fed the fish last night s/he did not go into a frenzy like the others, just stayed hovering in place, once in a while going back and forth across the lower back inch of the tank, making a kind of "munching" movement with his/her mouth (the kids now named the fish "Mr. Munchie"). This morning I was afraid the fish would be dead. However, it was swimming around with the others! We fed the fish, but Mr. Munchie did not seem able to get any of the food, even though we tried to direct some towards him/her. Eventually he retreated to the back bottom of the tank again. As I write this, he is swimming around the tank, moving fairly well, but more often than not the odd man out with the other rummynoses. His nose is red and I don't see any discolored patches, etc. He's the second smallest out of four rummies. All the other fish look very happy. Should I just observe, quarantine, or take back to the store (14 day guarantee)? I'm worried about neon tetra disease. P.S. Tank parameters are perfect, but the temperature of the tank was a bit high because I had a new heater that turned out to be un-calibrated. Over the past few days I have been gradually lowering the temp from 87 (I know, way too hot but they all seemed happy) to 79 degrees.