i guess it would depend on your lighting and what kind of plants you have. I don't use ferts and have less than 1 wpg and my plants are flourishing. I just have java moss, java lace fern, bolbitis, anacharis, anubias, and watersprite. All of mine are reproducing and quite green but they all are easy keepers and none are planted in the substrate. what are you intending to keep?