Yeah they should be insured for this and if they aren't that's on them. That's already such a vulnerable situation for you to find yourself in, to then get blamey sounds like they could use a bit more time in the tank themselves. Check your local laws around these types of establishments, often they are lumped in with spas, and see if they have legal requirements about insurance coverage. If they do, the insurance should cover it, and if they don't, report them.

This is honestly no different than accidentally vomiting in a taxi. They will charge you a fee, like 150$ where i am, to cover the cost of cleaning. Both are biohazards and both require a proper cleaning and sanitization, but no taxi driver is going to try and get you to replace, that's absurd, as is this. The 150$ amount is ostensibly their lost income from time and costs of cleaning.

Finally, they have presented an unrealized hypothetical as fact. They don't want to clean it. They say they don't think they can sanitize it but have they tried? They need to try and fail or point to official documentation from the manufacturer to back that up before they have a toenail to stand on. My money is on this: insurance will likely only cover the cost of sanitization, and not a replacement, again without something to back that up other than vibes or ick. I bet theres a cheap owner who doesnt want to make a claim on their policy and has hourly employees threatening to quit if assigned the task. Meanwhile they think they can get a brand new tank free out of this. That reimbursement for cleaning is going to be significantly less than 8000, and even if they reimbursed for loss, they'd depreciate it for age and wear, and it would still be high but less. But this amount is low enough (sub-10k) and the damage was accidental, not intentional, that any insurance company would have a hard time trying to hold you responsible for their costs of coverage.

I am not a lawyer but I've been in a situation where we had to claim the maximum our renters policy covered (100k!). Our insurance loved that we exhaustively documented everything and tried to go after the landlord for it as they were unambiguously responsible under the law, but insurance found multiple properties all with multiple leins and the owner had moved overseas, they gave up after a year. It costs them money, time, and effort to try so there needs to be a good chance of success on their part and theres both an upfront cost and diminishing returns over time.

I think your best bet is to research the laws and regulations around spas and just own them on the facts. Don't be above having a friend call them and ask a lot of questions like "I've never done this and have some concerns, and at some point ask them point blank about their insurance coverage. "What if I pass out or slip on a wet floor and need medical care, what does your insurance policy cover?" Present it like you assume they must have one you just want some reassurance about it.

Best of luck and I'm so sorry this happened to you. Screw these losers.