There is a REASON tadpoles have a naturally high mortality rate, not to mention this is just incredibly cruel to force them into such crammed conditions. I guarantee you most of them are suffocating from depleted oxygen as well lack of food (cannibalism likely through the roof)
Conditions like this also facilitate the problematic spread of infections or diseases that can not only harm these frogs directly, but jeopardizes the native amphibians and other wildlife whose habitats they end up being released into.