If you could, please post your tank size, and the ammonia, nitrite, and nitrate readings. If you don't know the size of the tank, click on the FishLore logo at the top of the screen and click on the conversion chart link. It will help you determine your tank size. It could be that the tank has not been maintained properly, and now that you've come along, changes in the routine were too much for the fish. It could possibly be old tank syndrome:
http://www.bestfish.com/oldtank.html
I highly recommend you get an Aquarium Pharmaceuticals Master Test kit which will include all the tests you will need right now. You can get them at most fish stores, but if you find the master test kit on the PetSmart website and copy the page, you can probably get it for about $13.50. Take the page with you, and they should honor the online price at your local PetSmart.