The 4 slots in your toolbox are for 4 different types of supplies for your tanks. Each slot can only be used for one type of supply. Slot 1 is for food, and the type of food will change when you research better food. Slot 2 is for medication; Slot 3 is for special chemicals; and Slot 4 is for eggs. The slots can hold one thing at a time. As you've already seen with medication, if you have something in a slot and want to buy something else that goes in that slot, you have to either use up what you have or it will be replaced when you purchase something else for that slot.