To raise slimes successfully you are to feed different species with different foods. Some slimes eat only certain fruits that are extremely hard to find, the others eat everything that you give them – from the most popular fruits and vegetables to meat and even other slimes. There are also very rare slimes who live in the distant locations and eat exclusive items that are extremely hard to find.
For example, the latest location from the 1.1.0 update brought the whole new location into the game – The Wilds. In this location only there is a very rare fruit called Kookadoba. It can be given only to the certain goo species. Additionally, you can feed slimes with Phase Lemon (grows only in The Ancient Ruins). An interesting thing about it is a full transparency. Through the tree with Phase Lemons you can go through, and lemons cannot be vacked in the standard way, like other species. To harvest this lemon, it is necessary to shoot a ripe fruit from tree branches by means of any other fruit. The thrown fruit disappears from this reality and one of phase lemons appears instead. Awesome mechanics! Prickle Pear is another common fruit that grows in the Glass Desert. It really resembles an ordinary purple pear with spikes on the peel. You can grow your own trees with prickle pears without leaving the garden. One tree can deliver many fruits for a few harvest seasons. Mint Mango is one more very popular, colorful green fruit that grows only in the Moss Blanket region. Besides these fruits, you should also look for Pogofruits, Cuberry, When it comes to the veggies in Slime Rancher, your choice will vary among Oca Oca (resembles a potato), Odd Onion (occasionally appears in your garden), Silver Parsnip (looks like a silver carrot), carrots, and the special Heart Beet that should be given to rock goos. Meat is another food source in the game. Its varieties are mostly different chickens with slightly different characteristics.
The hardest food to find in the game is called Gilded Ginger. This is a mythological vegetable that occasionally appears somewhere in the Glass Desert. Even developers from Monomi Park published the guide and official map how to find this really rare resource.