Change out about 75% of the water ASAP. Don't forget to add your water conditioner (Stress Coat, I think you said) along with 2 Tablespoons of ordinary table salt (without iodine is but don't stress this right now--use what you have), dissolved prior to adding.
Wait a few hours and change out another 50% of the water, again adding the Stress Coat and 1 Tablespoon of salt.
If you do this morning and night (the two water changes), you might save your fish. If not they will all surely die.
Changing the water will dilute the ammonia that is killing your fish. The salt will ease their gill function and promote production of their slime coat.
Change the water FIRST and then get those test results. The stuff that is critical to your success right now is
A bigger tank or tub
The biggest filter you can afford (there is no such thing as too much filtration)
The API test kit (you are going to have to test your water everyday and do water changes until toxicity is reduced to zero
A good water conditioner....Stress Coat isn't that great, it contains aloe which tends to gum up your filters and fish don't really benefit from it. I like Prime, but there are several other brands as well.
Don't buy any thing other that what I've suggested. If fact, take back anything you've purchased and ask for a refund, even if you've used some. Most of it it bunk.
Oh Wait. Keep the Pima Fix and Melafix...it's good stuff, but not right now.
Again, please listen to us--or better yet join a gold fish community like this one--the pet/fish store does not have your best interests at heart. They want to make money.