hi Mike, sorry to hear about your spring, it's really frustrating and sad how fast they can go downhill seemingly out of nowhere. One thing to consider is the moisture gradient of your enclosure. It seems like even though you can keep springs completely aquatic, if you keep them semi-terrestrially the terrestrial area *needs* to have some relatively dry areas. I don't know if it's just behavioral osmoregulation, or if there are diseases that they can fight better in dryer vs damper areas, or if they're extremely sensitive to changes in substrate chemistry/spoilage. In 2018 I actually had a very similar experience to what you're describing--two seemingly-healthy adult springs went from happy to lethargic to dead, each in the span of a couple days--and the only common factor was that I'd gone a bit crazy with misting each of their enclosures in the week or so leading up to their deaths, not leaving any dry areas. More recently, something similar started happening with a ruber I'd been keeping in a semi-aquatic setup. I moved it to a larger enclosure with the water area separated, and made sure to leave some parts of the land area more dry. It's been in there now for about 1.5 years with no issues so far! Again, I'm really sorry to hear about your spring. They're my favorite plethodontids (along with rubers), and it's really sad to lose one. I'll second josh's advice, though with the caveat to make sure to give a moisture gradient (I prefer dirt to paper towels when fridging sallies, but that's just me).