The plants in your tank are absorbing the ammonia your fish release, the excess nutrients, and the CO2. I wouldn't be surprised to find that your biological media probably doesn't have any beneficial bacteria on it, the plants are doing most of the work. Since you have found a way of keeping your plants alive, and successful, I'd say you're one of the few fortunate people who really only need to do water changes every 6 months, or so (according to an article I read, but which I can't currently find).