my plants were ok with just light but really didn't grow until I added fertilisers. This was ok for the hardy plants like the ambulia but nothing else was really growing steadily until I added co2. Now the plants are growing like crazy and they're all 'pearling'? at least half the day. I'd imagine it's better with a pure co2 input as well but the days I've lowered the ferts I've noticed there is less growth (yeah, i can see the difference in a day with co2 + ferts + light).