Actually, carbon will take up nitrates. The problem is that unless you have a massive amount of carbon, it won't make a dent in the readings of the average tank. That is, if you use the typical amount of carbon placed in one filter pad, the amount of nitrate it can uptake is within the margins of error for the home nitrate test kit. Also, the nitrate-carbon attraction is pretty weak, so when some other chemical comes along that may be more attracted to carbon than the nitrate is, that nitrate gets released back into the tank.
Carbon is pretty useless, except for taking medications out, I always take an exacto knife and cut open my filter pad and shake all the carbon out. The nice thing is that then I can use that same filter pad for several months instead of replacing it every few weeks like the manufacturer recommends. Hmmmm, the manufacturer recommends I buy a lot more of their product that is really necessary... coincidence?
There are a few other things that carbon does that I don't care for. Carbon adsorbs tannins. And while many people don't seem to like tannins leaching from their wood, the tannins do a bunch of really useful functions. Firstly, they bind to heavy metals, making those heavy metals a lot less toxic to the fish. This is really important is your water conditioner does not have a heavy metal neutralizer. The toxic levels of many heavy metals for people is many orders of magnitude higher than for fish, so water companies don't regulate all of them. For example, copper is very toxic to fish, by the time humans can taste copper in their drinking water, copper is still not at toxic levels for humans, but are like 500 times the toxic levels for the average fish. But, tannins bind up heavy metals keeping the fish safer from them. Also, in their bound up state, those heavy metals are more easily taken up by live plants. The tannins help plants get their micronutrients easier. Finally, for some species, the presence of tannins is a spawning trigger. So, with all these good functions, I don't want carbon taking tannins out of my water. Besides, the tinting is more natural anyway.
Another carbon does is, since adsorption is an equilibrium process, chemicals are always being adsorbed and desorbed on it all the time. That means that if you use the carbon to take medications out of the tank, you need to throw that carbon out very soon after. Because it will release some small amount of the medication over time.
Basically, stick to the water changes. Water changes do so many other useful functions, too, like refresh the buffering of the water, and it is like a breath of fresh air to you or I. Your fish will really respond to regular water change.
