Water changes do not decrease CO2. I will go out on a limb to say when your 10 angelfish become adults, your nitrates will go through the roof if you dont do water changes. I have still been trying to find balance in my tanks, but the nitrates still are going up too high. Without water changes my tank would become toxic with nitrates in short order. Still I dont know how minimal your changes are but I would suggest 10-25% per week, you can go higher or lower depending on how your nitrates increase. You also could go with the EI dosing and do the 50% water changes per week, then you wouldnt have to think as much about over dosaging. There would be a better and stable balance this way.
As for CO2 again, I dont use it either. CO2 is exchanged at the water surface and is maintained at some 2-3ppm roughly. Water changes are not going to affect that negatively. The water you are putting in will likely have the same CO2 levels.
If you want to have a tank without changing water, there are ways to do it, I am just not familiar with them.
http://theaquariumwiki.com/Walstad_method this will give you some information and ideas.