Aha, hm. Yeah, that makes sense.the temp change causes the volume of liquid to shrink, and can therefore cause suckback of water from your airlock into your beverage
Well, the water in my air locks is sanitized with no-rinse sanitizer... how long does the sanitizer stay active? I don't actually know what brand. I use Star San, now, but this was a white, powder sanitizer that came with my first brewing kit, unlabeled aside from "Use 1 tbs per gallon. No rinse necessary."
Will this be okay, or should I prioritize getting a solid stopper?
Edit: actually, how long does Star San stay active, too? I know it's an acid-based sanitizer, and my experience with finicky aquariums is that acid-water mixtures don't stay acidic for long, due to "buffering."
Edit 2: nevermind, found some great threads on Star San. Apparently, it has a long shelf life, even diluted. Nice. Not sure about the generic stuff currently in my airlocks, though.