Just reading an interesting article on tropica website: Co2 and light stimulate plant growth part 3 http://www.tropica.com/default.asp It looks at the respective effects of co2 and light on plant growth (not suprising). However they did an interesting experiment on growth rates of riccia: Table below. The percentages are growth rates per day. http://www.tropica.com/catalog/images/articles/akvaristik/UK_CO2_tabel1_large.jpg 5400 lux is meant to correlate with a well lit planted aquarium. There is a 3x increase in growth upping the co2 to 6.6mg/l, but upping it further to 35 mg/l only increases growth by 1% compared to before. Why then is 30ppm the recommended level for co2 in the planted aquarium if it has such a minimal effect? Would it not be better to cut down the co2 for a bit to make it last longer? The other question I had on my mind is whether 4hrs of 50w lighting will have the same effect as 8hrs of 25w lighting? Essentially is it intensity or duration of light that is more significant to growth rate?