Conophytums don't need much in the way of fertilizer; a light misting with a balanced water-soluble fertilizer, once per month during the growing season, should be sufficient for adult plants. I've experimented a bit with various commercial and home-brewed preparations intended to provide micronutrients and vitamins for plants, and have never seen any clear effects from them, positive or negative. Most soil mixes will provide plenty of micronutrients, without any special additives.

Be wary of natural fertilizers that contain large amounts of undecomposed organic matter, like blood meal, ground seaweed or earthworm castings. I've had serious problems with fungal growth after using these materials.