Fish food is currently only used in one quest, Ernest the Chicken. The only place for free-to-play players to find fish food is in Draynor Manor, where it spawns on the 1st2nd floor[UK] in a room with level 36 ghosts. Fish food can be used with poison that can be found in the kitchen of Draynor Manor.
Players, with the poisoned fish food, are then able to kill aggressive piranhas in the fountain in the south west grounds of Draynor Manor by using the poisoned fish food on the fountain. Once done, the fish food will disappear from the player's inventory, but the player then can retrieve the pressure gauge.