To the OP, if you don't mind my asking...what state are you in? Knowing that may help to pinpoint the issue as each state has different things dissolved in the water supply. In Southern FL our water had lots of salts, phosphates and silicates in it, plus we had to use a water softener on the well water going into the house, so we never used it for our fish tanks. We purchased a really good RO/DI hi-S unit by Kent Marine and our water was great after that. We had fresh, salt and reef tanks with that super clean water. The unit is pricey, but worth it when you want to keep fish.
I'm with fishfanatic, I don't believe it has anything at all to do with an un-cycled tank or ammonia. Unless there are a bunch of dead fish inside the well causing ammonia? Where would the ammonia come from straight out of the tap? My city water doesn't even test for traces of ammonia or chlorine and you know all the chemicals they add to that. I have always added a few fish to a brand new tank once it's heated up to the proper temp, filter is running, airstone is running and I've added water conditioner and aquarium salt and tested the water parameters to make sure they are in line with the fish going into the tank. In our 18+ years of setting up tanks that's how we have always started up a tank. I've never yet lost a single fish due to cycling a tank.
Without a professional water test, I would probably give up on the well water and use either an RO/DI filter or buy spring water or distilled water to keep fish in and of course, use a good water conditioner with ANY water you use for fish. I also swear by adding aquarium salt per directions to new tanks since it helps add electrolytes for the health of your fish. Livebearers and goldfish do well with added aquarium salt. Your well water can have many other contaminates that city tap water does not. Here in Arkansas, many wells have sulphur and they stink really bad. Without a professional water sample being tested from your tap, it's tough to say what is killing off the fish. We have city water but I just filled our new 30 gallon with distilled water to give it a better start. Our city tap water has lots of phosphates and silicates in it and grows algae like crazy. And yes, I also know that the distilling process can leach copper into the water which is why my water conditioner neutralizes it and other heavy metals.
I'm not sure if any of this will help you out, but best of luck to you and your future fish! I hope you find a solution.
