Oxygen is the variable that kills fish, and it does it overnight. A lake can be fine at 4 p.m. and lethal at 5 a.m., and nobody is on the water to see it.

Continuous optical DO with temperature, logging at fifteen-minute intervals and alerting when readings cross a threshold you set. Deployable at a single depth or on a multi-depth string to capture stratification and hypolimnetic oxygen loss through the season.