I have a 2.5 g, and EVERY fish I have put into it has died. The first fish was a fish I brought home to try to rescue that was dying. I expected him to die, and he did. Then I rinsed out the tank in hot water. The fish that died, died of an accident. He had been caught in coral and returned to my store. I didn't think there was anything wrong with him besides his injuries.
Then there was Hidalgo. Who I brought home thinking he was stressed. I put him into a heated 10 g, but once I realized he was sick I moved him into the 2.5 g for treatment. He died within a couple days of being put into the tank. It wasn't a surprise though, granted he seemed healthier in the 10 g than the 2.5 g.
Then I soaked it in bleach water for several hours, let it dry outside, soaked it again in bleach water for several hours, let it dry outside. I rinsed it out with warm water, and left it to dry in the bathroom with the fan on. It's sat empty for a month, then last night I broke down and got Poe. He's been at PetSmart for a long time. I thought he was gone (dead or purchased), I went and got Hudson, saw him there and went back for him last night. I rinsed out the tank with hot water again and scrubbed it with a paper towel.
Last night after acclimation and being put into this new tank I noticed Poe acting weird. My boyfriend and me actually got into a huge fight over it, because he was mad I spent an hour staring at Poe trying to figure out what was wrong. He was resting on the bottom 1/2 on his side and clamped up. He wasn't like that in his cup. He wouldn't really move unless I moved the tank a lot. Then he started swimming around normally, so I thought he was just adjusting. I went to sleep woke up this morning and he was still alive. I came downstairs, got on the computer, have been on here for about two hours, went up to check on him... and he's dead! Seriously? He survives months of crappy care, a columnaris outbreak a month ago, and bad water conditions. He was the only fish on the shelf that wasn't bloated or having trouble swimming. I was so torn taking him home and leaving others, but I felt he'd earned his forever home. And, then this!
Could it be the tank? Could there be something wrong with it? I just can't figure out how three fish have all died within a day or two of being put into the tank. Granted, two were sick, but this last one wasn't showing any signs of illness. Even in death he still looks great.