Will an opened bottle of water conditioner for my fish aquarium expire?

What do you think "expire" means? People food is required by federal law to have a date on it, but the date does not have to mean anything. For example, you can buy "280 million year old salt" and it expires in three years. That is obviously meaningless. Careful investigation turns up a theory that in three years some of the chemical in the plastic bottle might leach into the product and the manufacturer denies legal liability after that.

What is this "water conditioner" of which you speak? I have spent years trying to find straight answers on water filtration and it's awfully hard to find. In nature, water is conditioned by accident. So why do you have to pay for it to happen? And if you don't know, why are you paying?

Bottom line: you need to learn your business, and I mean from professionals, not strangers on the internet.

It will say on the package somewhere, and if not you can probably google the brand.