Overfeeding fish in tanks does shorten their lifespan. Fish suffer the results of overweight just like people do. Fatty livers and kidney problems are the most likely results. Inapproprite foods are as bad as too much food (high protein diets for grazers, etc.)
But the commonest short term result is just poor water parameters from excess waste/undigested food. How harmfull that is long term will depend on the upkeep given the tank by the keeper.
But the commonest short term result is just poor water parameters from excess waste/undigested food. How harmfull that is long term will depend on the upkeep given the tank by the keeper.