Sorry to hear about the loss. That’s really tough.
If the LFS contaminated the tank, causing the fail, I think it’s their fault. But I would keep this in perspective: nobody set out to hurt the fish. Rarely does someone come to work to say, “I think I’m going to be careless and hurt some animals today.” The outcome stinks, but that doesn’t necessarily mean anyone intended to hurt the fish.
If it were me, I’d make a list of what I need to be up and running. List your fish and confirm the LFS will replace or pay you for nominal retail value if they can’t source them all in a timely way. Maybe an ICP test to check your water first? Then what else do you realistically need to get running again? The alternative, if you can trust them, is to ask them to come in, clean up, and restart your tank for you, maintaining it regularly for a certain amount of time and helping to ensure all the new livestock survives. That’s a big ask, and you may not want that, but it’s something to think about.
I’m not sure I would go the court route myself. You never know what a judge would decide. The LFS probably wouldn’t even argue it - they’d likely call their insurer to deal with it, so you wouldn’t get any satisfaction there. A lot of jurisdictions consider pets to be property, and they’ve offered to replace the property, so it sounds like an uphill battle to get much else to me.