FL PERSON, I am brand new to this Forum, so been reading your post here with interest. Happy to hear that things have now worked out for you, and that terrible stain has disappeared. What a nightmare... I too would have been sickened by that whole ordeal.
As someone new to being a Hot Tub Owner, I have to say that sadly, I have found that when you do buy one a lot of stuff seems to be Trial & Error... Manufacturers & Dealers just don't seem to provide Buyers with enough info. I have been reading up like crazy in the last month (and hence how I found this Forum).
Clearly many others have similar issues when it comes to new Ownership, and exactly why websites like this one get so much traffic.
Our Hot Tub was installed, filled & initially set up chemically by the Dealer... After that it was as I say a matter of Trial & Error (it is now about 1 month old and FINALLY I am feeling that I am getting the hang of things when it comes to water chemistry). Again thanks to websites like this one.
Unfortunately, Trial & Error means mistakes are made.
I think your Dealer should have been clearer with you on installation, filling & set up... IMHO you should have been told not to do anything with the tub until after the electrical connection was completed. As well "chemicals should have only been added once the pumps were running / water circulating". That info appears in various places for our own Hot Tub (First Time Set Up Instructions - User Manual - and even printed on the side of the various chemical containers)
As well... This Advice from HTNJ gave me pause...
I'm not saying it's the reason, but I always pre-mix my chemicals before adding them into the tub.
I have a large size mason jar with lid, maybe 32oz.
Add the chemicals to that, scoop up some spa water, lid, swirl to mix, pour slowly into spa.
Everything I've read about chemicals says the following:
1- Add Chemicals to Water... NOT the other way round if you choose to premix / dilute them
Although, some Hot Tubs this step is not required, and chemicals can be added straight into the tub without dilution (READ / CONSULT you Spa Dealer for further info)
2- NEVER mix various Dry Chemicals together (although not sure if that is what HTNJ was saying here).
Chemicals should be added one at a time. And sometimes they need to be added with an intentional interval (again READ the Directions for use on their containers and / or CONSULT with your Spa Dealer).