Sorry lol I'm quiet new to fish aquariums
We were all new at one time and made same or worse mistakes, so no worries. Plus, that is what this forum is for, to help you make less mistakes or correct them when you do.
The most important thing right now is for you to have 0 nitrites, stop messing with the PH and address the black on the fantail.
I really recommend you do a 25% or more PWC daily until the nitrites are 0 and you get all the ph altering chemicals out of your water. Nitrite is only slightly less poisonous than ammonia for your fish.
This will also help your goldfish if it does have finrot, but I now suspect it is showing symptoms of nitrite poisoning.
The pet shop says it's due to using well water instead of city water which what most of the treatments are meant for.
Are they talking about nitrites? if so, that is untrue.
If its about ph, don't worry the PH. Stop treatments and let the fish acclimate to your ph from your tap water. Goldies like a higher ph anyway.
Also, what is the ph of your tap water compared to your aquarium?
If its the same, do just let your fish acclimate to the ph you have normally.
It is almost impossible to keep ph higher or lower than what comes out of your tap water. Fish will acclimate to whatever yours is, but can get stressed and even die when the ph level rises or lowers too quickly.
Also, albino corys are fine in a cooler temperature, which is better for the health of the fantail. I would advise you too set the heater at maybe 68, no more.
Should I go ahead and cycle my 2nd tank and move the fish once it's ready and my lvls have evend out some.
YES
And best of luck!