I have a new, ten-gallon tank. It is heated to 78 degrees f, has aquarium rock substrate and a live plant. It also has a fairly inexpensive filtration system, and is lighted incandescently. I got the tank less than a week ago and over the past few days almost all of my fish have died.
At first I just wanted feeder guppies, with two lyretail mollies and two snails. The first day, all but two of the feeder guppies died. I got more feeder guppies the next day, figuring the others had died because the store I got them from treated them roughly and they were in bad shape from the start. I also got two panda cories and a 2 male and 3 female fancy guppies. Since then everything has died except a few of the feeder guppies, a pair of fancy guppies and one lyretail mollie.
For the most part all of them seemed fine (eating, exploring, etc) until they developed symptoms extremely quickly. One minute they would be fine and the next they would be having a hard time swimming. They would float vertically in the water and just let the current take them, and then they would die. I did notice somewhat fractured fins on some of the ones that died.
I also have another 10 gallon tank but it is in my boyfriends room. It didn't have nearly the problems that I am having. While the number of fish was kind of large, we have had quite a few in the other tank in his room (10 or so) without these kinds of problems. The water temperature is ok, but i haven't tested other water parameters because I don't have a testing kit. It seems fast for the water parameters to go south especially since nothing else seems to be amiss (no coloration changes, plants still healthy, etc). I hope this is enough information and that someone can give me some insight.