Hey Nik (Southern greeting),

Out bullfrogs and brown frogs just go to the bottom of the barrels and
berm ponds. There is no sand, only pond liner or barrel bottom (They
are scraping the bottom of the barrel :}) They do fine there and come
up awake in the spring. If you have any plants in your pond, they may
find a way into or under them. Our main pond frogs generally go under
the planters, which have been lifted by cinder blocks. If we put sand
in the main pond, the koi would dig it up.

Jim