Newbie fish disaster at hand

My recent attempt to maintain 1 little goldfish has been frought with
disaster despite a fair amount of reading and help from reputable
petshops. I purchased a single gallon tank with gravel filter and
airstone. Despite what I'd read about a larger (10 gal) per fish tank,
the pet stores assured me this was not a problem as long as I
maintained it. My SIL has the exact same tank and 1 goldfish and has
never had a problem. Within 2 days the water (tap with proper drops to
remove metals/chlorine etc) turned yellow and started to stink of
fish. This progressed until after 5 days it looked horrible (the
petshop said this was normal cycling and it would clear). After 6 days
I changed about 50% of the water. The yellowing began again, quickly.
And "goldie" began developing black spots (ammonia burns?). I took the
water to be tested. Ammonia high--like death zone high, pH hi
alkaline(bright pink on the test strip) and water level hard (I have a
water softner so I don't understand that one). I was given MelaFix, ph
acidfier and an amonia remover. I used all three last night.

ph: after two separate doses of 2 drops per gallon, pH had not moved.
I continued to give drops and test every hour. no change

Ammonia: I ran out of test strips so I am taking the water in tonight
for a test again.

Fish: is eating (I cut WAY back on the food---this was my problem)but
looks sad and black is not better and now on some fins. Why is it
spreading??

I fear I am torturing this small fish. Would a tetra have been better?

Would appreciate constructive advice.

M