Cannot choose any potion in procedure???

Hey modders! Today i was making a potion sack! While i was making the item i had to make a procedure to make sure i could only store potions inside this sack! Basically pressing the right mouse button will make you open a gui with 4 slots, one of them is made to insert the potion! after that you will need to press a button (i like buttons in guis) to check if the item is a potion and then see if the first available potion slot to then insert it! The poblem? well you see i was starting with making the first if statement and i made so it would check the object in the place were potions are inserted and then if it equaled to a potion, well i was trying to find the potion and i was surprised to see no potions other than the simple water ones! Will i be able to choose at least one type of potion or will i haft to wait to have made a lot of my own potions for this object to be useful?