I do/have tested for nitrates and they come up 0 in the tap water. Only time I see a 0 in fact. Like many, I can keep the tank (I only have the one) down to 5 and 10 ppm
by testing frequently and religiously doing my water changes. If I see it at 20 I start to increase my volume and frequency of water changes.
The 7.0 out of the tap is a mixed blessing because it is done by running our water through a tank of calcite out in the garage. So clearly the 7.0 comes at a cost. It is done in this way so that the naturally low pH water will not deteriorate our pipes and fixtures. If something goes wrong with that system, it will not be 7.0 out of the tap and of course the composition of the water will change. Hence the frequent tap water checks for pH.
While my water has some pluses, I certainly would prefer softer water with fewer dissolved minerals. I do not have a general hardness test kit, but do plan to get one. I don't have to be a rocket scientist to know that my water contains dissolved minerals. I'm not planning on messing with the hardness, or pH for that matter; but it would be nice to know what it is I'm actually dealing with. My tank water stays at 7.2 to 7.4. Letting our water sit in a bucket for 24 hours renders a 6.4 to 6.8 pH.
(I didn't know when we decided that we wanted a few tetras, that I was going to have to become a water expert, lol!)
We are on a very complex aquifer here in the Sierra foothills, and it is difficult to say just where our water comes from. NO ONE, including some highly paid scientists fully understand the hydrology here. I've heard that by the time it gets to the well, the water is thousands of years old. I'm not sure I buy that, but one thing is sure, it tests 0 for ammonia, nitrite, and nitrate out of the tap.
But to keep this on topic, because I don't fully understand my water source and because it is a small household system that we are entirely responsible for running, I tend to see carbon in my tank filter as insurance until or unless I can get an RO
system. I lean towards the thinking that good quality activated carbon, properly rinsed, used, and changed, will do no harm. I don't mind spending the money, though I'd rather buy my choice of carbon in bulk and put it in my own media bag than to be beholding to the filter manufacturer.
I already have heard about the HITH carbon theory; but beyond that, I was curious why people are so against activated carbon, so of course I googled. I'm paraphrasing, but I found out it is considered old school and a sign of laziness to use it except to remove meds! Of course, I first and foremost believe in water changes. I tend to see activated carbon as insurance against unknown trace impurities in my water source, and certainly not as a way to keep my water clean without changing it.