I think the liquid does expire in about a year. I bought a hagen test kit and about a year later my tank's parameters sucked...all over the place, not constant at all.
I had no idea what was going on. Even after huge water changes everything was high. My filter media had been running for years, so I thought what the heck was going on.....? After getting lots of mulm from friends and doing massive water changes daily, I took a water sample (from a few tanks and the tap) to a LFS and they said the water was fine. So I went home and tested my water again and I got terrible results. So I bought an API test kit and the problem was solved haha....aka I don't think I had a problem at all, just my test kit expired.
So, at least the hagen test kit does expire, that is all I can come up with for a reason that my water parameters would totally bounce all over the place.
