RO/DI water, and why tap water fails

What's actually in tap water

Drinking water standards exist to stop water making people ill. They are not concerned with whether it will grow coral, and several things that are entirely acceptable in a glass are a slow problem in a reef:

• Chlorine and chloramine. Deliberately added, and lethal to your filter bacteria and gill tissue. This is the one part a conditioner genuinely fixes — but chloramine is harder to break than chlorine and needs a proper carbon block to remove rather than neutralize.
• Phosphate. Often added on purpose, as orthophosphate, to stop lead and copper leaching from old pipes. In a reef it inhibits calcification and feeds exactly the algae you are fighting.
• Silicate. Naturally present nearly everywhere and removed by nothing you would put in a tank. It is what diatoms build their shells from.
• Nitrate. Common in agricultural areas, and it sets a floor you cannot water-change below — a point the water change calculator makes concrete.
• Copper and other metals. From your own plumbing as much as the supply. Copper is acutely toxic to every invertebrate you keep, and it accumulates.

What each stage removes

• Sediment. Grit and rust. Its whole job is protecting what comes next.
• Carbon block. Chlorine and chloramine. Also protecting what comes next — chlorine destroys an RO membrane permanently, so this is the cartridge never to be late on.
• RO membrane. The workhorse: rejects 90–98% of everything dissolved, and sends the rest to the drain.
• DI resin. The polisher, taking the last few ppm to zero. It is also the consumable whose cost this page is about.

That ordering matters more than it looks. Each stage exists partly to protect the next one, and the two cheap cartridges at the front are what keep the two expensive ones alive.

What a gallon really costs

Here is the part the other guides skip. DI resin does not have a lifespan in gallons — it has a fixed ion capacity. It exchanges a certain quantity of dissolved solids and then it is spent, so how many gallons that represents depends entirely on how loaded the water reaching it is. Which depends on your tap water, and on how much of it the membrane caught:

Computed by this site's engine at 95% membrane rejection, with the last column at 98%. Capacity is 2,000 ppm-gallons for a standard 10-inch cartridge, from SpectraPure's published figure of about 100 gallons at 20 ppm feed; cartridge priced at $20. A manufacturer number for one product line, so read it as a budget rather than a guarantee — resin chemistry and which ions you have both move it. The DI resin life calculator runs this for your own tap reading, membrane and usage, and adds the tap water and the consumables to get an all-in cost per gallon.

Same system, same membrane, 10× the running cost. A reefer on 50 ppm municipal water gets 800 gallons from a cartridge and pays 2.5¢ a gallon in resin. One on 500 ppm well water gets 80 gallons and pays 25¢. Neither of them bought the wrong equipment; they live in different places. This is why "RO/DI costs about X a gallon" is not a fact anyone can quote at you.

The membrane is the lever, not the resin

You cannot change your tap water. You can change how much of it gets past the membrane, and the effect is larger than it sounds because DI life depends on what is left rather than what was removed.

Going from a typical 95% membrane to a healthy 98% one leaves 2% behind instead of 5% — so your resin lasts exactly 2.5× longer, whatever your tap water is. A tired 90% membrane against a healthy one is a factor of 5. That is the difference between changing resin monthly and changing it twice a year.

What actually protects rejection, in order of how often it is the problem:

• Replace the carbon block on time. Chlorine damage is permanent and it shows up as creeping TDS.
• Feed pressure. Membranes are specified around 60 psi; below about 50 they underperform badly, and a booster pump pays for itself on low-pressure supplies.
• Temperature. Cold water is rejected less efficiently. Winter output dropping is normal, not a fault.
• Flush it. A flush valve, used briefly at each run, keeps the membrane surface clear.

The one tool that matters

An inline dual TDS meter, reading before and after the DI stage, turns all of the above from guesswork into a gauge. It costs less than one wasted cartridge and it tells you three things nothing else will:

• Your membrane's rejection rate, from the tap and post-membrane readings — measure both and you have the number.
• When the DI is actually spent, rather than when the color-change resin looks tired, which is not the same moment.
• That your water is still good today. The failure mode people get caught by is a silently exhausted cartridge quietly feeding the tank the very things the system exists to remove.

The rule for the final reading is genuinely absolute: 0 TDS, not 1. A reading of 1 means the resin has started to let ions through and the ones it releases first are frequently the ones you least want.

Do freshwater tanks need it?

Mostly no, and it is worth saying plainly on a page that has just spent 2,000 words on why tap water is a problem. A community tropical tank on treated tap water is completely normal and works fine. The cases where RO genuinely earns its place:

• Soft-water species — discus, cardinals, wild-caught fish — where you need water softer than your tap will ever be.
• Shrimp, especially caridina, where GH is the parameter that matters and precision beats whatever the utility sends.
• Tap water with high nitrate, which sets a floor no water change can get below.
• Anywhere the supply varies — starting from zero and remineralizing to a recipe gives you the same water every week, which is the real benefit.

Note that RO water on its own is not aquarium water: it has no buffering and no minerals, so a tank filled with it has nothing holding pH steady. Freshwater use means remineralizing to a target, not filling with pure water — see the parameters chart for what you would be aiming at.

What matters when buying

Everything above points at a short list of specs, and it is a different list from the one the marketing leads with:

• Membrane quality and rated rejection — the single number that determines your running cost.
• Waste ratio, if you are metered or environmentally minded.
• A dual inline TDS meter, included or added immediately.
• Standard 10-inch housings, so replacement cartridges are a commodity rather than a proprietary subscription.
• Rated GPD you will actually use. A 100 GPD unit is not better than a 75 if you make 25 gallons a month; higher-flow membranes sometimes reject slightly less.

Notably absent: stage count. A "6-stage" system is not better than a 5-stage one by virtue of the number — the extra stage is often a second DI or an inline carbon polish, both fine, neither transformative.

What you'll be told, and what's true

• "RO/DI costs about X per gallon." Not a quotable figure. The resin alone swings 10× between soft and hard supplies, before waste water and before the state of your membrane. Anyone quoting a single number is quoting their own postcode.
• "Buy the DI resin with the biggest capacity." The membrane in front of it matters far more. Improving rejection from 95% to 98% multiplies resin life by 2.5 at any tap TDS — more than most capacity upgrades, for less money.
• "A dechlorinator makes tap water safe." Safe from chlorine, yes, and for most freshwater tanks that is genuinely enough. It removes nothing though — and for a reef, or for top-off water that leaves its whole dissolved load behind as it evaporates, "neutralized" and "gone" are not the same word.
• "More stages means better water." The output is 0 TDS or it is not. A well-maintained four-stage system beats a neglected seven-stage one every time, and the meter is the only thing that knows which you have.

Common questions

Can I just use a dechlorinator instead?

For chlorine and chloramine, a conditioner genuinely works — that part is real chemistry. What it does not do is remove anything: the phosphate, silicate, nitrate, copper and everything else are still in the tank afterwards, because a conditioner binds or neutralizes a couple of specific things rather than filtering the water. On a freshwater community tank that is usually fine. On a reef, phosphate and silicate are precisely the inputs you spend money removing later.

Is 0 TDS really necessary?

For mixing salt and for top-off, yes — and it is worth understanding why the standard is so absolute. Every gallon of top-off water leaves its dissolved solids behind when it evaporates, so anything in it accumulates permanently. A tank topped off with 10 TDS water is being handed a small dose of silicate and phosphate every single day, forever. That is a different situation from a one-off water change, and it is why reefers are stricter about top-off water than about anything else.

How often do I change the filters?

Sediment and carbon on time — typically every six months, sooner on dirty or high-chlorine supplies — because their job is protecting the membrane, and they fail silently. DI resin when your TDS meter says so, not on a schedule. The membrane when its rejection rate drops, which the meter also tells you. Replacing carbon late is the most expensive mistake available: chlorine that gets past it destroys the membrane permanently.

Is it cheaper than buying RO/DI from the store?

Usually, though less dramatically than people assume once waste water is counted. Store water runs roughly $0.50–0.75 a gallon in most places. Making your own costs the consumables plus the water bill on both the product and the waste — on soft water that lands well under a tenth of store pricing, on hard water the gap narrows considerably. The stronger arguments are convenience and control: you can test what you are putting in, and you are not carrying jugs.