R.I.P. Nemo

What the hell!? Nemo met the same odd demise as Gil. The boy at Petsmart said it sounds like Swimmers Bladder but gave no good insight. I have been on the internet trying to research and still can't figure out what is going on. We have a 12 gallon tank, with a filter, light, treated water reading 'safe' levels on our meter. Its clean, cold and tastefully decorated, I might add. Fishey, the first fish, is thriving and doing great. Perhaps Fishey wants to live alone and is beating up the new guys when we aren't looking? who knows. We may give the second fish thing one more try. If the next one dies, I give up!