I got results back from water tests, and it was a combination of ammonia and adding fish too quickly. I've been changing the water in the tank and the person I talked to said it should be ready to hold more fish in a few weeks. Thanks for your input, guys.
Chico (grey tabby cat) - 15 years
Draco (white ferret) - 3 years
Bellatrix (seal point ferret) - 3 years