Hi Brad, sorry to hear that.
Obviously you know now that axolotls require more care... Pet stores often give wrong information, the same thing happened to me however I luckily did more research before disaster struck.
Living in Sydney, your axololt would have just experienced a lot of hot weather. People quote different temperature ranges that are acceptable, I try and keep mine below at least 20 degrees celcius by freezing 1.25 litre soft drink bottles and floating them in the tank, and by keeping these on a constant rotation, I can keep the tank cool. It's also really important that the temperature doesn't fluctuate drastically.
Also you have likely read that the water needs to be within certain limits in terms of ammonia, nitrate, nitrite, ph, that the water needs to be cycled (*knows that some clever person will arrive soon to post the cycling guide which I can't find just now...*) and that the tank should be free of anything small such as gravel that can cause impaction and kill the unlucky axolotl that swallows it. A filter for the tank is a good idea, however make sure it doesn't create too much water flow which will stress an axolotl. I do 20% water changes weekly and remove any waste/uneaten food ASAP before it can foul the water. Axolotls also need dark hidey holes and a decent size tank.
Also I try and give my axolotl a varied diet consisting mostly of bloodworms, and pellets and beef heart occasionally. The bloodworms and beef heart are available at pet/aquarium stores but the beef heart should be served as a treat only. There are a whole heap of other things you can feed them but those foods are just readily available to me and within my poor uni student budget.
There are just my initial thoughts and it's entirely possible that I've missed or forgotten something crucial, hopefully some more experienced person will come along and put their two cents in. Don't give up though, just do some research and when you're ready and set up, look into getting another little gilled friend. :happy: At least you recognise that there was a problem and you're trying to fix it for your future pets.
You should go back to your pet store and kindly point out where you were given incorrect advice and pass on some of the things you have discovered... you never know, they probably just have no idea how wrong they are and you may save a little axolotl life.
Good luck!