He is too big to get into my filter. My betta was like 2 inches.
Nothing elseYou mention cories. Is there anything else in the tank? If I lose a fish overnight chances are I will never know because there will be nothing left by morning due to other fish, snails and shrimp (depending on which tank).
While it is true that dead fish can contribute to ammonia spikes I would not expect to see one from a single dead fish in a mature tank.