Best Nitrate Filter for Well Water (Whole House & Under-Sink Options)

When a well test comes back with nitrate at 10 or more, two filters actually take it out. A whole-house nitrate filter treats every faucet, shower and hose bib in the house. An under-sink reverse osmosis system treats the water at one drinking-water faucet in the kitchen.

The whole-house filter looks just like a water softener. The only difference is the resin inside, which is made to pull out nitrate. For a well over the limit it is the one I recommend, and one tank is enough for most homes. Below is how to choose between the two, what size you need, what else has to go in first, and what arrives at your door.

Whole house or under the sink?

Pick by where the water has to be safe. A whole-house nitrate filter treats every faucet in the house; an under-sink RO treats one. In the video I put it this way: with the whole-house system "you can take a drink from any faucet in the house and it will be nitrate free."

Reverse osmosis (RO) uses a semi-permeable membrane to physically reject dissolved contaminants, including nitrates. It does not reject all of it, and how much depends on the water temperature and water pressure. I tell callers a reverse osmosis "might give you 50 percent reduction, maybe 70 under optimal conditions, whereas the whole house filter will give you a hundred percent reduction." At 20 parts per million, half only gets you to 10, right at the limit, so an RO only makes sense when the level is a little over 10. Whichever you buy, test the treated water after it goes in; the Minnesota Department of Health gives the same advice.

One thing neither system changes: the Minnesota Department of Health recommends that babies under 6 months not drink the well water even with treatment in the house, in case the treatment fails. For a baby, use bottled water.

If the nitrate is well over 10, or you want the shower and the bathroom sinks covered too, the whole-house filter is the answer. What each one costs to run every year is in the cost and maintenance guide.

What size nitrate filter do I need?

One tank with 1.5 cubic feet of nitrate resin is enough for most homes. I keep the answer short when people ask: "The 1.5's usually enough, unless you have like a huge family."

A caller in September had typed a well test and "four people in the house" into an AI tool that morning, and it said to buy a bigger nitrate tank than the one we had suggested. I told the caller it "works for any size home," and why: "You're not using water all the time." A family uses a lot of water at times, but not every second of the day, and the valve cleans the resin after a set number of gallons, not on a clock.

The part that does get sized to your house is the water softener ahead of it. That caller's water was 34 grains hard with 4 people, so the softener had to be a 64,000 grain one, while a 48,000 grain one would not have been enough. The nitrate tank stayed the same.

We built the 2510SXT version because a customer "wanted all his equipment to match," so it is there for matching, not because most homes need it. Before you order, send me a lab report that shows nitrate, hardness and sulfate. Sulfate competes with nitrate for the resin, and a high number changes what I tell you. If you do not have those numbers yet, the $199 well water test covers all three; the nitrate-only test does not.

What has to go in first

If your water is hard, a water softener goes in first, then the nitrate filter. In the video I explain why: "You have to remove all the hardness in the water before it goes through the nitrate resin or the nitrate resin will start pulling out the hardness as well and diminish the performance of the nitrate filter." Iron and manganese have to come out ahead of it too.

On a well with acidic water as well, the usual line is "a neutralizer, a water softener, and then a nitrate filter." The full treatment order for removing nitrates is in its own guide, with what each tank does.

Nitrate result in hand?

Text me your water test

Send a photo of the lab report with how many people live there. Aidan will tell you which system fits and whether a softener has to go in first.

Text Aidan443-277-2204 Call800-460-5810Aidan

8 AM to 5 PM Eastern, seven days a week. A real person, not a call center.

What ships in the box

The tank ships empty, and the resin gets poured in at install. For the Fleck 5600SXT nitrate filter, what ships is:

• the 10 x 54 inch mineral tank with the Fleck 5600SXT valve, the bypass valve pre-installed on it
• the nitrate-selective resin in separate boxes, and a fill funnel for loading it
• the salt tank with a safety float

Drain tubing is not included; any hardware store has the 1/2 inch tubing. The box has little install guidance, because the makers expect a plumber or a water treatment dealer to put it in.

Who installs it

I recommend having a plumber put it in, and I'm a phone call away for your plumber. For one nitrate tank that is about 2 to 3 hours and about $800. Plumbing it in is an inlet and an outlet; the part that takes planning is the 1/2 inch drain line to a floor drain, standpipe or utility sink, with an air gap. The valve also needs an outlet nearby. With a softener going in ahead of it, that is two tanks and 3 to 4 hours, and a plumber who has installed water treatment before is worth finding. If you'd rather do it yourself, text me with any questions.

Why it needs nitrate resin

The resin is what makes a nitrate filter work, and it has to be made for nitrate. A standard anion resin prefers sulfate. When it fills up, it can release the nitrate it caught, so the treated water comes out with more nitrate than went in. Penn State Extension describes exactly that, and water treatment people call it nitrate dumping.

The Fleck 5600SXT and the 10 GPM system both use ResinTech SIR-100-HP, a nitrate-selective resin. Its maker says it "eliminates the possibility of nitrate dumping." That is also my answer when someone asks about a cheaper filter online: "it's a lot less expensive because, you know what, it doesn't work." We tried other nitrate filters and gizmos, and the whole-house Fleck 5600SXT nitrate filter was the one that worked. The resin is cleaned with ordinary pellet salt, the same as a softener's; my advice is "just keep salt in the salt tank."

Frequently asked questions