I lost a mosquito fish as well on the same spot, maybe jumped trying to
catch something in the air.

Anyone know how to "glue" river rocks together? Do you think I can use
aquarium silicone to attach some rocks to the rims of the barrel?
(plastic liner)

Those with the cascade barrels, do you tilt the upper barrels slightly?
I did and at some places the water is closer to the rim of the barrel,
so I think I need to build some kind of net or dam to fence off those
areas.