Just swapped tanks - here’s a picture. You can see some spots on the left where the bond between the gasket & film is starting to peel away.
I usually check the bottom on all my tanks before starting a print and sometimes tanks will have little, small bubbles like this but they don’t increase in size (or at least not in the same time frame).
The bubbles on this Tough 2000 tank have been getting bigger over time and one of them is getting really close to the edge so it’s time to change. I’ve never actually had a tank fail and spill (knock on wood), so I don’t actually know exactly what the failure mode is, but I presume if these bubbles get bigger, it will start leaking. Probably not all at once, but slowly.