Toxins in Aquarium water?

Hi, ok I have just set up a fresh water aquarium. The tank has been brought up to temp and filters, heaters, air-ration have been running for 36 hours. I picked up two Pictus Catfish and both died the next day. They were properly acclimated to the tank. A couple days later I picked up a Fiddler Crab and... dead the next day. I know since it's a new tank there is not an established Nitrogen cycle going yet, but I have been testing the waters pH, Ammonia, Nitrite and Nitrate levels and they are all at zero PPM. The pH level is at 7.4. What is going on here? is there something toxic in our water that does not affect the test I performed? We are on a well so there should be no chemicals that tap water will have. Please help if you can Thanks, Tom.