Figured
water change intervals wouldn't be dictated at all by co2 injection. In fact, I would think it'd give you more time between intervals due to the
nitrate intake from the increased growth of plants IE water changes less often?
That would seem to make sense, but it hasn't panned out that way in my experience. Just the opposite. My high-energy, high-tech tanks demand far more water changes and other maintenance.
Essentially, the plants grow a lot faster in a high-tech tank (assuming the same lighting and temperature). To make sure the plants don't become deficient with this fast growth, fertilizers must be added. It's way too difficult to add the perfect amount of every fertilizer for a given high-tech tank, so the easy solution is to just add a lot of everything. But, to make sure those fertilizers don't build up to toxic levels, regular large water changes need to be done.
Furthermore, if lighting is bright (typical of most CO2 injected tanks), organic waste has a tendency to trigger algae. (Organic waste build up doesn't seem to cause the same problems in a low-energy, low-tech tank.) Plants create organic waste through the loss of old leaves (and possibly other secretions and such). If plants are growing super-fast, they grow a lot more leaves and create a lot more organic waste. Therefore, to keep organic waste build up from triggering algae growth in a high-tech tank, regular large water changes need to be done.
The need for water changes in a high-tech
planted tank is not all about nitrate build up.
The way around the need for so much maintenance in a CO2 enriched tank (as ruud stated above) is to lower the light and temp enough so that the tank works more like a low-tech one. I've done this myself, and it does reduce the amount of maintenance required (especially trimming). In my experience, the plants still grow a smidgeon faster, but they retain much of their high-tech growth forms.
Personally, I think low-light, CO2 tanks are an under-utilized approach.
