The best RO/DI systems, by what a gallon costs on your water

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An RO/DI system's box carries a stage count and a GPD. Neither decides what the water costs you. The running cost is set by two specs the box mentions in passing — the membrane's rejection and the DI cartridge's capacity — and by what the replacement cartridges cost, and those three things make a different system cheapest on soft water than on hard. Every figure below is the RO/DI cost calculator's engine run per system, at 25 gallons a month, using each maker's own rejection, capacity, waste ratio and cartridge prices.

Which RO/DI system for your water

For a tank using 25 gallons a month, the cheapest system to run all-in — resin, tap water at the system's waste ratio, and the year's prefilters and membrane — at each tap reading, and the cheapest of the lines with a maker behind them. Read your tap TDS off a meter or your water report.

25 gal/month; tap water at $11 per 1,000 gal including sewer; each system's own rejection, DI capacity, waste ratio and cartridge prices (Aug 2026, rounded, from the makers' stores — not Amazon). "Best overall" is the cheapest of the BRS, SpectraPure and AquaFX lines. Amazon links are affiliate links; the rest earn us nothing. Your reading goes through the calculator.

At a glance

Rejection as the maker states it, or the conservative end of a stated range; DI as a multiple of a standard 10-inch cartridge's 2,000 ppm-gallons (the BRS 6-stage runs two standard cartridges; the RO Buddie's compact inline DI is our assumption at half). "Cartridges / yr" is prefilters at the maker's interval plus the membrane amortized — not the DI, which depends on your water and is in the tables above.

How we ranked them

Three things, in this order. What a gallon costs on your water — from the two specs that set it, computed per system rather than asserted. What the GPD really is — rated at what pressure, and what it makes in winter. Then what it is like to own: standard 10-inch housings or proprietary cartridges, a gauge and a flush valve, a maker who still sells parts in five years. Stage count is not on the list: a fifth carbon stage on soft water is a cartridge to buy, and a second carbon block on chloramine water is essential, and the number alone cannot tell you which. Price bands are from the specialist retailers on the date at the top; commission is not a factor, and three of the six links earn us nothing.

The picks

Best overall for municipal water

Bulk Reef Supply 4 Stage Value 75 GPD

A reef on ordinary city water (under about 250 ppm, chlorine rather than chloramine): a real FilmTec membrane, standard 10-inch cartridges you can buy anywhere, and the cheapest consumables of any branded system.

Membrane

96 % rejection

DI capacity

2,000 ppm·gal (standard)

Waste

4:1

In the box

Pressure gauge; flush kit and inline TDS meter on the "Plus"

Cartridges

DI $22 · prefilters $22 × 2/yr · membrane $36 / 2.5 yr

Street price

$150–200 (2026-08)

Each size, by the box and by the physics

System	Box says	Winter reality	Resin at 200 ppm	All-in · a year
75 GPD · 4 stages →	75 GPD at 50 psi, 77 °F	38 GPD at 55 °F, 40 psi — 16 h for 25 gal	250 gal a cartridge · 304 days	34¢ at 200 ppm · 47¢ at 500 · $58/yr in cartridges

"Box says" is the maker's GPD at its own rating conditions (source: Bulk Reef Supply product page and instructions). "Winter reality" is the FilmTec temperature correction and net driving pressure at 55 °F and 40 psi on 200 ppm water, and the hours that takes to make 25 gallons. Resin is gallons per DI cartridge on 200 ppm tap at this system's rejection and capacity, and the days that is at 25 gal/month. All-in is resin + tap water at the waste ratio + a year of prefilters and membrane, per gallon, at 200 and 500 ppm.

Why it's here

• A FilmTec BW60 membrane — the one everyone else's "compatible" membrane is copying — rated 96–99 % rejection.
• Standard 10-inch housings, so every cartridge is a commodity: a sediment and carbon set for about $22, a DI refill for about $22.
• BRS publishes its cartridge prices and change intervals, which is why this line's running cost can be computed rather than guessed.

What to know

• One carbon block — on chloramine-treated water you want two, which is the 6-stage below.
• A 4:1 waste ratio, the ordinary figure; the SpectraPure runs 2:1.
• No booster pump: under 50 psi the 75 GPD is a number on a box.

Bulk Reef Supply →

Best for chloramine water and big tanks

Bulk Reef Supply 6 Stage Deluxe Plus 150 GPD Water Saver

A bigger reef, or any tank on chloramine-treated water: the second carbon block is the chloramine answer, the second membrane halves the waste, and dual DI means the second cartridge catches what the first lets slip.

Membrane

96 % rejection

DI capacity

4,000 ppm·gal (high-capacity)

Waste

2:1

In the box

Dual carbon blocks, dual DI, two membranes in series, pressure gauge, flush kit, inline TDS meter

Cartridges

DI $44 · prefilters $38 × 2/yr · membrane $72 / 2.5 yr

Street price

$330–400 (2026-08)

Each size, by the box and by the physics

System	Box says	Winter reality	Resin at 200 ppm	All-in · a year
150 GPD · 6 stages →	150 GPD at 50 psi, 77 °F	77 GPD at 55 °F, 40 psi — 8 h for 25 gal	500 gal a cartridge · 608 days	47¢ at 200 ppm · 60¢ at 500 · $105/yr in cartridges

"Box says" is the maker's GPD at its own rating conditions (source: Bulk Reef Supply product page). "Winter reality" is the FilmTec temperature correction and net driving pressure at 55 °F and 40 psi on 200 ppm water, and the hours that takes to make 25 gallons. Resin is gallons per DI cartridge on 200 ppm tap at this system's rejection and capacity, and the days that is at 25 gal/month. All-in is resin + tap water at the waste ratio + a year of prefilters and membrane, per gallon, at 200 and 500 ppm.

Why it's here

• Two membranes in series: the second processes the first's waste, so 150 GPD at a 2:1 ratio instead of 75 at 4:1.
• Two carbon blocks in series — the only reliable protection for a membrane on chloramine.
• Dual DI: run the second as a guard and change the first when the meter moves, which is what the calculator's "0 not 1" rule needs.

What to know

• Twice the cartridges to buy: a prefilter set is about $38 and two DI refills about $44, so a low-usage tank pays for capacity it never uses.
• Big: six housings across, plus the meter.
• The same 96–99 % FilmTec rejection as the 4-stage — you are buying stages and water saving, not a better membrane.

Bulk Reef Supply →

Best on hard water — the membrane and the resin

SpectraPure MaxCap 90 GPD

Well water and hard taps, where resin is most of the bill: a 99 % membrane leaves half the ions a 98 % one does, and a 3.5× DI cartridge holds them — the two levers the calculator says matter, in one box.

Membrane

99 % rejection

DI capacity

7,000 ppm·gal (high-capacity)

Waste

2:1

In the box

0.5 µm sediment and carbon, 99 % SpectraSelect membrane, MaxCap high-capacity DI, three-probe TDS meter, manual flush, 2:1 restrictor

Cartridges

DI $65 · prefilters $40 × 2/yr · membrane $80 / 3 yr

Street price

$330–420 (2026-08)

Each size, by the box and by the physics

System	Box says	Winter reality	Resin at 200 ppm	All-in · a year
90 GPD · 5 stages →	90 GPD at 60 psi, 77 °F	38 GPD at 55 °F, 40 psi — 16 h for 25 gal	3,500 gal a cartridge · 4,258 days	41¢ at 200 ppm · 43¢ at 500 · $107/yr in cartridges

"Box says" is the maker's GPD at its own rating conditions (source: SpectraPure product pages and specialist retailer listings). "Winter reality" is the FilmTec temperature correction and net driving pressure at 55 °F and 40 psi on 200 ppm water, and the hours that takes to make 25 gallons. Resin is gallons per DI cartridge on 200 ppm tap at this system's rejection and capacity, and the days that is at 25 gal/month. All-in is resin + tap water at the waste ratio + a year of prefilters and membrane, per gallon, at 200 and 500 ppm.

Why it's here

• A 99 % rejection membrane: against a 96 % FilmTec that is four times less reaching the resin, so the resin lasts four times longer — pinned by the calculator's own arithmetic below.
• The MaxCap DI cartridge is the high-capacity anchor the whole site uses (about 3.5× a standard cartridge).
• A 2:1 waste ratio out of the box, and a three-probe TDS meter that shows the membrane's rejection live.

What to know

• The dearest system and the dearest cartridges here: a MaxCap DI is about $65, a membrane about $80.
• Proprietary-grade cartridges — standard 10-inch housings, but the good ones come from SpectraPure.
• Rated at 60 psi; on a 40 psi supply it needs the booster like everything else.

SaltwaterAquarium.com → Maker's page →

Most reliable — the twenty-year system

AquaFX Barracuda 100 GPD

The keeper who wants a system to outlast three tanks: forum reports of Barracudas at fifteen and twenty years on nothing but cartridges are not unusual, and the US-made housings are why.

Membrane

96 % rejection

DI capacity

2,000 ppm·gal (standard)

Waste

4:1

In the box

1 µm sediment, chlorine/VOC carbon block, TFC membrane, mixed-bed DI; gauge on the Platinum

Cartridges

DI $19 · prefilters $30 × 2/yr · membrane $55 / 2.5 yr

Street price

$200–280 (2026-08)

Each size, by the box and by the physics

System	Box says	Winter reality	Resin at 200 ppm	All-in · a year
100 GPD · 4 stages →	100 GPD at 65 psi, 77 °F	39 GPD at 55 °F, 40 psi — 15 h for 25 gal	250 gal a cartridge · 304 days	40¢ at 200 ppm · 52¢ at 500 · $82/yr in cartridges

"Box says" is the maker's GPD at its own rating conditions (source: AquaFX product page and owner's manual). "Winter reality" is the FilmTec temperature correction and net driving pressure at 55 °F and 40 psi on 200 ppm water, and the hours that takes to make 25 gallons. Resin is gallons per DI cartridge on 200 ppm tap at this system's rejection and capacity, and the days that is at 25 gal/month. All-in is resin + tap water at the waste ratio + a year of prefilters and membrane, per gallon, at 200 and 500 ppm.

Why it's here

• Built on standard 10-inch housings in Florida; the brackets, clips and fittings are the parts that fail on cheaper systems and these do not.
• A replacement set of sediment, carbon and DI for about $49 from the maker.
• 100 GPD at a stated 65 psi — read that rating as generous below 65 psi.

What to know

• AquaFX states "over 90 %" rejection for the HI-S membrane; we use 96 %, the conservative reading of a TFC element, and the calculator's Measured option is how to know.
• 4:1 waste.
• The base model has no gauge or flush valve; the Platinum adds them for more.

See on Amazon → Maker's page →

Best budget system

LiquaGen 5 Stage 75 GPD

A first reef on soft municipal water, where the calculator says resin is a small share of the bill and the difference between a good and a great membrane is a few dollars a year.

Membrane

96 % rejection

DI capacity

2,000 ppm·gal (standard)

Waste

4:1

In the box

Sediment, two carbon stages, membrane, DI; pressure gauge and handheld TDS meter

Cartridges

DI $16 · prefilters $24 × 2/yr · membrane $30 / 2.5 yr

Street price

$100–140 (2026-08)

Each size, by the box and by the physics

System	Box says	Winter reality	Resin at 200 ppm	All-in · a year
75 GPD · 5 stages →	75 GPD at 60 psi, 77 °F	32 GPD at 55 °F, 40 psi — 19 h for 25 gal	250 gal a cartridge · 304 days	32¢ at 200 ppm · 42¢ at 500 · $60/yr in cartridges

"Box says" is the maker's GPD at its own rating conditions (source: LiquaGen product page and Amazon listing). "Winter reality" is the FilmTec temperature correction and net driving pressure at 55 °F and 40 psi on 200 ppm water, and the hours that takes to make 25 gallons. Resin is gallons per DI cartridge on 200 ppm tap at this system's rejection and capacity, and the days that is at 25 gal/month. All-in is resin + tap water at the waste ratio + a year of prefilters and membrane, per gallon, at 200 and 500 ppm.

Why it's here

• Standard 10-inch housings with a real gauge, for about the price of two premium DI cartridges.
• Two carbon stages, which covers chloramine better than most budget systems.
• Cheap cartridges, and the housings take anyone's.

What to know

• LiquaGen states the membrane is "for city water not exceeding 300 TDS" — that is an honest limit, and above it the resin bill climbs fast.
• A GE-made membrane rather than FilmTec; rejection is not stated, so we use 96 %.
• Fittings and clips are where budget systems leak; check every push-fit before the first fill.

See on Amazon → Maker's page →

Best for a nano — and honest about its DI

Aquatic Life RO Buddie 50 GPD 4-stage

A nano or a small fish-only tank using a few gallons a week, where the calculator says a full-size system never pays back against store water — this one might.

Membrane

96 % rejection

DI capacity

1,000 ppm·gal (compact)

Waste

4:1

In the box

Compact proprietary cartridges: sediment, carbon, membrane, color-changing DI

Cartridges

DI $18 · prefilters $26 × 2/yr · membrane $35 / 2.5 yr

Street price

$60–90 (2026-08)

Each size, by the box and by the physics

System	Box says	Winter reality	Resin at 200 ppm	All-in · a year
50 GPD · 4 stages →	50 GPD at 65 psi, 77 °F	20 GPD at 55 °F, 40 psi — 31 h for 25 gal	125 gal a cartridge · 152 days	42¢ at 200 ppm · 64¢ at 500 · $66/yr in cartridges

"Box says" is the maker's GPD at its own rating conditions (source: Aquatic Life product page). "Winter reality" is the FilmTec temperature correction and net driving pressure at 55 °F and 40 psi on 200 ppm water, and the hours that takes to make 25 gallons. Resin is gallons per DI cartridge on 200 ppm tap at this system's rejection and capacity, and the days that is at 25 gal/month. All-in is resin + tap water at the waste ratio + a year of prefilters and membrane, per gallon, at 200 and 500 ppm.

Why it's here

• Small enough to live under a sink, and the cheapest branded RO/DI made.
• Rated honestly: Aquatic Life states the 50 GPD is at 65 psi and 77 °F, which almost no one else prints.

What to know

• Proprietary compact cartridges, so you buy Aquatic Life's or nothing; the DI is a small inline cartridge we count at half a standard one's capacity.
• No gauge, no flush valve, no meter — you are running blind unless you add a handheld TDS meter, which you should.
• 50 GPD at 65 psi and 77 °F is 25 GPD at 40 psi and 55 °F; the table below does the arithmetic.

See on Amazon → Maker's page →

What "75 GPD" actually makes

A membrane's rating is at 77 °F and the maker's test pressure — 50 psi for the FilmTec elements in the BRS systems, 65 for AquaFX and Aquatic Life. Cold water is thicker and crosses the membrane more slowly (FilmTec publishes the correction: 65 % of rated flow at 55 °F), and every psi below the rating is a psi that does less work — at 40 psi on 200 ppm feed, 79 % of the rated net driving pressure. Multiply them and a 75 GPD membrane in a cold garage on a weak supply makes 38 GPD, which is 16 hours for 25 gallons instead of the 8 the box implies. A booster pump to 80 psi takes the same membrane to 79 GPD in the same garage.

Gallons a day and hours to make 25 gallons, at each system's own rated pressure; 200 ppm feed. The two 65 psi-rated systems lose the most at 50 psi because their rating assumed more pressure than most homes have — the same membrane rated at 50 psi would print a smaller number on the box and make the same water.

The two specs that matter: rejection and DI capacity

Resin life is capacity divided by what reaches the resin, and what reaches the resin is your tap TDS times what the membrane lets through. On 300 ppm water a 96 % membrane passes 12 ppm to the DI; a 99 % membrane passes 3. That is 4× the resin life from the membrane alone, and the SpectraPure's MaxCap cartridge holds about 3.5× a standard one's ions on top: 167 gallons a cartridge against 2,333 on the same water, 14× — the calculator's arithmetic, pinned by a test. Which is why the dearest system on the page is the cheapest to run above about 600 ppm: from there up, resin is most of the bill, and the SpectraPure barely buys any.

What each system costs to run, per gallon

Every line of the bill for each system, at 200 ppm and 25 gallons a month — and how long the system takes to pay for itself against buying water at the store's 62¢ a gallon, using the middle of its price band:

200 ppm tap, 25 gal/month, $11 per 1,000 gal of tap water; DI cartridges at each maker's price and capacity; payback on the middle of the street-price band against 62¢/gal store water. Nothing here is an Amazon price.

Do I need a booster pump?

If the gauge reads under 50 psi with the system running, yes, and it is the best money on the page after the meter. A membrane below its rated pressure makes less water and rejects less — the net driving pressure that pushes water through is what leaves the salts behind — so a 40 psi supply costs you production, resin life and membrane life at once. A booster ($60–120) runs the membrane at 80 psi regardless of the house: on the 55 °F, 40 psi garage above it turns 38 GPD into 79. The LiquaGen 6-stage and the Ice Cap systems bundle one; on the others it is an add-on that fits any 10-inch system. Well water on a pressure tank that cycles between 30 and 50 psi is the classic case.

How RO/DI systems fail

• The carbon block, silently. Its job is to stop chlorine reaching the membrane, and when it is spent the chlorine goes straight through and damages the membrane permanently — a rejection that creeps from 98 % to 90 % over a season is a carbon block that ran too long. Change it on the interval, not on looks.
• The DI, silently. Resin does not fade; it holds everything and then lets go, and the first things through are the silicate and phosphate a reef least wants. A 1 on the meter after the DI is a spent cartridge, not a rounding error. Dual DI (the BRS 6-stage) exists so the second catches what the first releases.
• The fittings. A push-fit tube not seated to the second click weeps at 60 psi, and it weeps onto the floor while you are at work. Push every tube home, pull to check, and put the system in a tray.
• The membrane, from its own waste. Minerals concentrate on the reject side and scale the surface; a flush valve run for thirty seconds at each use clears them, and the systems without one age faster.
• Freezing. A garage system with water in the housings and a cold night. Drain it, or heat the space.

What we'd skip

• Buying by GPD. A 150 GPD system on a 40 psi supply in winter is a 75; and unless you fill a reservoir in a hurry, the day length is not the constraint anyway — a 50 GPD system makes 25 gallons overnight.
• A system with no gauge and no meter. The gauge tells you whether you need a booster; the inline meter tells you when the DI is spent and what the membrane is doing. Without them you are changing cartridges by superstition.
• "Compatible" membranes with no stated rejection. The calculator cannot cost a membrane that only says "75 GPD"; neither can you. FilmTec prints its rejection; buy the one that does.
• A drinking-water RO with a pressure tank, for the reef. No DI stage, and the tank's air bladder is a home for bacteria; the water is fine to drink and adds 10–20 ppm to the sump.
• Extra carbon stages on chlorine-only water. One block handles chlorine. The second is for chloramine, which your water report states; if it is not there, the fifth stage is a cartridge you buy twice a year for nothing.

Common questions

How often do I replace the membrane?

When its rejection drops, not on a calendar — and only a TDS meter reading before and after the membrane tells you that, which is the calculator's Measured option. On chlorine-free feed with the carbon block changed on time, a FilmTec membrane runs two to four years; the systems here amortize it over two and a half. What kills one early is chlorine that got past a spent carbon block, and that damage is permanent, which is why the prefilter change interval is the one to keep.

Can I drink the water, or use one system for both?

RO water, yes — it is what the under-sink drinking systems make. RO/DI water, no: deionized water is stripped of the minerals that make water taste like water, and the resin is not food-grade. The neat answer is a tee before the DI stage with a small pressurized tank for the kitchen, which every 10-inch-housing system here can be plumbed for; the RO Buddie cannot.

My store sells RO/DI at 5 TDS. Is that good enough?

For a water change, probably; for top-off, no, and the reason is the arithmetic on the calculator: every gallon of top-off leaves its solids behind when it evaporates, so a 5 TDS top-off is a permanent, daily dose of whatever those five parts are — usually silicate and phosphate, the two things a reef spends money removing. Home-made water at 0 is not fussiness. It is the only way the top-off adds nothing.