Thanks for correcting me. It’s called rock wool. 
I have only one in rock wool and it’s bunching onions. It probably wont grow very big but I’m experimenting.
I don’t feed it anything - it takes water from the wick. Though I’m thinking of adding some epsom salt into the water - I have had great results in the past with plants in regular soil.
Do you think epsom salt should work in the C&G?