Eating food is probably the quickest way to get low level experience for your Magikarp. Every minute a new berry is spawned into the tank and simply tap the food and Magikarp will eat it, getting more experience as you go. Some Magikarp will even have a special ability that increases the experience that is obtained from eahc of the food.
The Food appears with different rarities, with it varying based on the current stat of the berry. As standard, Oran Berry would be the most common but, if powered up to be stronger than others, the weaker Berry will become most common.
Every 25 levels you raise a piece of food, you will receive a Support Candy.