They can supposedly live several years in captivity, but they do seem to just drop dead without warning. I'm guessing we're losing the ones who would die in brumating, because even though we don't brumate them, they seem to slow down, stop eating, and sleep more in the winter, and we lost one last year as well, and apparently deaths in brumating, for both captive and wild frogs, are common.
The other three appear fine, the water tests OK (or, at least, it would be Ok if they were freshwater fish), and their diet hasn't changed (nor has their diet's diet changed), so hopefully they'll be OK.