Carbon filters are not meant for the purpose that you are using them. You do not necessarily want to purify your water, but remove the dissolved solids--2 very different things. Carbon filters are generally meant to filter out solids in precip form, not dissolved solids. It is probably doing very little, if anything to remove dissolved solids. Carbon filters are used a lot for drinking water because it will filter out some things bad for humans, like arsenic and also various odors and tastes that water with high levels of minerals may have, although most of these are not harmful, they just don't taste good. While it does filter out a lot of things bad for humans, there are other things it does not filter out, like lead and flouride (which is harmful in large doses) that it does not filter out and must be removed with an RO. A carbon filter is not the type of filter to use for growing.
And like I said in the last post, even if you are running hydro, you may not need "special" water. And if you do, you are going to need something different than a carbon filter.