Concur with previous posts. Curious as to what you actually mean by "high" nitrates? Some people might consider 20ppm as "high" but it is not. As was mentioned, the weekly partial water change removes/dilutes nitrates and unless something is biologically wrong in the aquarium this will suffice. Plants also consume nitrates, and anaerobic bacteria in the substrate, but it is the pwc that most keeps them in balance. Anything below 40ppm is considered to be basically safe, but as 1077 pointed out some fish do better with lower nitrates than this, and most authors recommend keeping them below 20ppm which will occur with weekly pwc. I have heavily stocked aquaria and they are fairly heavily planted, and the nitrates are constant at 5-10ppm, and I do a 40-50% pwc every week.
Hardness depends upon your tap water and any substances in the tank that might raise or lower it (calcium-base rocks and gravel raise it, peat filtration lowers it). Like pH, the hardness tends to lower very slightly over time due to the normal biological processes, and here again the weekly pwc keeps it in balance.