This is just a theoretical question, after just posting in a D&D topic I tried to figure out how long a 8.5fl.oz / 250ml bottle of prime lasts if you have only a 20gallon tank with weekly 60% water changes.
That is 12 gallons changed, and you only need about 1/4 cap, as a full cap works for 50 gallons.
A full cap holds 5ml. So the 1/4 cap is 1.25ml. 250ml of the full bottle will last for 200 weekly water changes, that is roughly 4 years.
And that brought me to the initial question of the topic: how long is Prime usable after opening? Will it ever go bad? I tried googling but can't seem to find the answer, i also don't see an expiration date on my bottle.
Again, this is just a curiosity question. No way a bottle of prime would last 4 years in my house. With all the tanks I got and the frequent water changes, I need a new bottle about every 4 months