We have a very large freshwater tank at work that has been the home to 10 pumpkinseeds, 5 shiners, and 4 large goldfish for almost two years. Today, I found 8 of the ten pumpkinseeds dead, and another that doesn't look so good, and I have no clue why. The water hadn't been changed in a while, but we've been keeping the filter clean...What could have caused only the sunfish to die? And all at once? They seemed healthy enough. I did a complete water change today, but the other sunfish still doesn't look so good. Any ideas?