Because they are more active at night then day time.
That mean they sleep during the day and play at night, Frogs
don't just croak at night. Normally that is just when you hear
them. Only the males croak and they are trying to attract a
They don't croak when the water stagnates. They croak when it is
mating season, in which case they are calling to a female.
unless by croak you mean die, and in which case
they will die when the water stagnates because it is unhealthy. It
also means you need to re-phrase your question.