I have lost fish in a large system in which I had propagating tanks using 4 hours of indirect sunlight. As I was using 64 square feet of tank with a 6" water de[th, it was easy to saturate some of these low level light demanding softies, On one occasion, while vacationing for two weeks in the Smokie Mountains for Autumn Roilage, a power outage with the local co-op caused a low level oxygen reading in the water coloum to kill all fish. No invetebrae were hurt, howevfer, there was much stress to the system. Luckily, this occurred the day before I returned home from vacation. No fish had decomposwed after the short time after death.
Unless you monitore dissolved oxygen levels levels 24/7, you will nwever know for sure.
Keep the Peace,Spread the Faith.
Patrick.