your not feeding the rock persay, your feeding the life on it. if your tank is fishless you will need to feed the "rock" aka all the bacteria and microfauna that inhabits the rock. just about any foods will work. regular fish foods etc. in an amount low enough not to promote nuiscance algaes but to keep the bacteria alive and the microfauna alive. if you dont feed anything to a fishless thank the micro fauna and bacteria WILL slowly starve. this will reduce biodiversity, and lower bacterial colony densities, making a spike more likely on the influx of nutrients, aka the addition of a fish and its foods.