I pre-filter my well water through a now discontinued API Tap Water Filter that is filled with API Nitra-Zorb, followed by an inline activated carbon filter. I get about 200g before I recharge with a brine solution. It works well for me:
There are inline nitrate filters available, but there use life may be somewhat short and I was not able to successfully recharge. Depending on your filter, you may be able to use pouches of API Nitra-Zorb to lower tank nitrates. Great if you have a canister filter or an Aquaclear type HOB. It is rechargeable several times with salt water, making it fairly cost effective. It's use life is compromised as detritus coats the resin pellets so you want to place it last in the filter water flow following good/fine mechanical and/or biological filtration. You might use two pouches, one in the filter and another in recharge solution and to alternate every week or two.
Note: I am not affiliated with API and there are other de-nitrate resin products out there.
I once tried using rain water, but with acid rain (pollution) and and asphalt shingled roof, I found that the water was not suitable.
You might find an RO or RO/DI system that can work for you (I simply ruled it out in my case).
As you know, keeping tank nitrates low is also key. Good housekeeping along with plants will assist. Most plants prefer ammonia as their N2 source and this is good, but because of your source water nitrate issue, you will also want (especially floating) plants known to readily handle nitrates. Anacharis comes to mind, but there are others as well.
I hope this helps some. Good luck and keep posting.
Tank on,
-Mike
