Had your exact problem, I got an RO unit for softening. After the recent rain I started getting 40mg/l at the tap! Before I moved I had some nitrate but nothing like this!
These are things I've done over the years,
Resins,
Nitrate remover, like Fluval labs. I used this product with ok results. It works and can be recharged. You need salt to re charge it, (marine salt) ok if you have a reef or brackish system. Otherwise it's an extra cost, small salt boxes are around £7-10 not much money but loads considering how much you'll use the salt.
Fluval Lab Series Nitrate Remover - 150g | Charterhouse Aquatics
It's ok though, can be used in filters so it makes it easy to pre filter water in a bucket before adding to your system. It's ok for 1-2 smaller systems only otherwise it is very time consuming. I think you get 10-15 recharges then it's spent. You must monitor like crazy if it used in a tank, that's why I preferred to pre filter, eventually I got bored of that. It's important to note production alongside test results, after a while a pattern will emerge, this becomes dead handy!
Nitrate cartridges like this,
https://www.flickr.com/photos/92891884@N04/13567240655/
It's the blue canister.
These things work great but, you'll only get about 3000 litres worth, depending on the cartridge capacity and the initial NO3 reading. They are classed to remove x amount TDS no3 from the water. This cartridge and 40mg/l no3 gives me 2053 litres nitrate free water, it's the first one I've used, it's great but powder is cheaper. I'll explain powder in a second,
Or RO cutting. This method is ok but, if you have 30ppm nitrate and cut 50:50 you get 15ppm nitrate. Not fool proof but much better. Be careful with RO, it can't be used neat, it must be re mineralised. Tap water is ok for this, I did this for many years, it's ok for all but the most sensitive species. You basically get an equal division of everything in the water, anything you can measure and whatever else is left. I used this method for a while, but you need a bit extra to modify your water to anything but straight divisions. For really soft water you need Kh powder.
You will need a TDS meter if you use RO, absolutely necessary, fortunately they are cheap.
Another option, just use RO and GH/Kh powders, this is where I've ended up after all these years, it's easiest and safest, it guarantees quality water. It's a bit of a headache at first but the difference is obvious instantly.
This all of course depends on the quality of your water. If it is soft, low GH/Kh, cutting it with RO will mean you need to buffer it back up to a usable source anyway (this is how I ended up here) you buy the Kh powder because you need a bit here or there, same thing with GH powder, so you start thinking, why use this cursed tap water at all? And that nitrate filter, they retail about £20 each, you may only need one a year, that's not much. If you expand or need more they can be pricey. Haven't finished it using it yet but I will not replace it, I'll expand my DI resin bed instead.
This powder thing is only a recent transition for me and I've done lots of various methods over the years trying to combat poor quality tap water. Personally I think with an RO unit and some selected powders you can keep any fish in the shop, providing you have the correct tank etc. it's taken the best part of a decade though trying different things.
While you need to replace pre filters on an RO normally every 6 months, that's only about £10, the DI resin replacement depends on membrane quality and efficiency and also the DI vessel size (250ml is ok for 2-3 normal size tanks)
This is sold in all different sizes so you just buy what you need for a year.
The membrane itself can last 4-5 years if it is well maintained, flush loop on RO unit sorts this out simple enough. The membrane prices fluctuate in line with capacity, more GPD=more £ to buy.
75gpd will run four systems JUST, unless you get silly with water storage. It's best to upsize a bit because they run slow in the cold.
Those are some of your options!