After the last fish death, which also occurred just after rain, I started to think that perhaps the rain was splashing off the surrounding environment and getting into my fish tank through sneakiness.
The shrub turns out to be viburnum tinus, and sadly it seems, is toxic to fish.
So right now I'm in the process of moving the experimental system to pastures less green.
I realize this has not been the first time I've felt I had the solution, but hopefully this poison plant might mark the end of the fiasco that has been my first batch of aquaponics fish.
There are eleven silver perch left out of what turned out to be twenty two.
Interestingly, as a result of seeing them in a container, I discovered some are twice the size of others.