A quality aquarium model is probably going to be the best bet. Those folk are very passionate about their hobby, it's not as if they're settling for inaccurate devices.
There's a reason Bulk Reef Supply is the premier RO water filter brand for cannabis despite focusing on aquariums.
Same rules apply across hydro, aquariums, and just about every hobby. Don't buy junk, and don't buy into marketing. There's a reason there are dozens if not hundreds of cannabis nutrient brands, and 90% of it is the same as what can be bought at home depot. Before Jacks 3-2-1, it was Peter's Tomato dry amendments. Same stuff, new branding.