One of my guess is this...
Insecticide spray is oil base. Therefore it is floating.
My top tank have the overflow hole on water level, thefore the insecticide enters the overflow hole to the bottom tank.

-> Bottom tank have the most insecticide, therefore heavy casualty on bottom tank.

-> Heavy casualties causes NH4 spike causing some top tank CRS to die but not as bad. (2 on top vs 20 at the bottom)