What Is RO Water? | Reverse Osmosis Explained

If you keep aquariums long enough, you’ll hear people talk about “RO water” like it’s a magic ingredient. In a way, it is: reverse osmosis water gives you a clean, predictable starting point so you can build the exact water your fish, shrimp, corals, and plants prefer. This guide explains what RO water is, how reverse osmosis works, when you need it (and when you don’t), how to set up a system, and how to remineralize RO water safely for aquarium use. Everything is written in simple, friendly terms for beginners who want clear answers.

What Is RO Water?

The simple definition

RO water is water that has been pushed through a semi‑permeable membrane to remove most dissolved substances. The process is called reverse osmosis (RO). The clean water that passes through the membrane is called “permeate.” The leftover, more concentrated stream is called “waste,” “brine,” or “reject.”

How reverse osmosis works

Imagine a very fine filter with microscopic pores that allow water molecules through but block most dissolved salts, metals, and impurities. When you apply pressure (from household water pressure or a booster pump), water is forced through the membrane. The membrane rejects a high percentage of contaminants (often 90–99%), and the brine carries those impurities away to the drain.

What RO removes well

An RO system (with proper prefilters) can dramatically reduce:

• Hardness minerals (calcium, magnesium)
• Alkalinity (carbonates and bicarbonates)
• Heavy metals (copper, lead)
• Nitrates, phosphates, and silicates (important for algae and diatoms)
• Sodium and many other ions that contribute to TDS (total dissolved solids)
• Chlorine and chloramine (these are removed by carbon blocks before the membrane)

What RO doesn’t always remove

Some dissolved gases (like CO2) can pass through the membrane. Certain very small organic molecules may partially pass. The membrane can reduce bacteria and viruses significantly, but a standard home RO unit is not certified as a sterilizer. Treat RO as very low-mineral water, not as sterile water. Always keep storage containers clean and covered.

RO, RO/DI, Distilled, and Bottled Water: What’s the Difference?

RO vs RO/DI (RODI)

RO/DI adds a deionization stage after the membrane. DI resin “polishes” remaining ions to get TDS as close to zero as possible. Reef aquarists usually prefer RODI because even small amounts of phosphate or silicate can fuel algae and irritate corals. Freshwater hobbyists can use RO or RODI; both work, but RODI makes parameters more consistent when your tap water is challenging.

Distilled vs RO

Distilled water is made by boiling water and condensing the steam, leaving most contaminants behind. It’s very pure but energy-intensive. RO uses pressure instead of heat, is more efficient at home scale, and is usually cheaper per gallon. For aquarium use, both can be used and remineralized. Many hobbyists find RO/DI more practical to produce at home.

What about bottled water?

“Purified” bottled water may be RO, distilled, or a blend. Labels are often vague. Mineral content can vary by brand and batch. If you must use bottled, choose a brand that lists TDS and mineral content, or test it yourself. For consistency and cost, a home RO unit usually wins.

Why Aquarists Use RO Water

Precise control over GH, KH, and pH

With RO, you start near zero minerals, then add exactly what your fish and plants need. This gives you reliable general hardness (GH), carbonate hardness (KH), and pH. Stable, targeted water reduces stress, improves breeding success, and makes plants happier.

Soft-water species and specialty setups

Some fish evolved in very soft, low-mineral waters (blackwater and rainforest streams). Examples include discus, wild angelfish, Apistogramma, many tetras, and some dwarf shrimp (e.g., Caridina cantonensis like crystal reds). RO makes it easy to achieve the soft, slightly acidic conditions they prefer.

Marine and reef aquariums

Salt mixes assume you’re starting with pure water. Tap water often contains phosphate, nitrate, copper, or silicate that can trigger algae or harm invertebrates. That’s why reef keepers almost always use RODI for mixing saltwater and for freshwater top-off in auto top-off systems (ATO).

When tap water is a problem

If your tap is extremely hard, has high nitrate or silicate, or comes with chloramine (common in many cities), RO prevents a lot of headaches. It can also eliminate variables if your water supply changes seasonally.

Do You Need RO Water?

Situations where you might not need it

Many hardy community fish (livebearers, danios, many barbs, most farmed tetras) adapt well to conditioned tap water. If your tap is moderate hardness, low nitrate, and you do consistent maintenance, you might do great without RO.

Situations where you probably should use it

• Breeding soft-water fish, wild-caught species, or sensitive Caridina shrimp
• Reef tanks or marine fish/invertebrate systems
• Plant-heavy aquascapes that need precise nutrient and hardness control
• Tap water with high TDS, heavy metals (like copper from old pipes), nitrate, phosphate, or persistent algae issues

A quick decision checklist

• Is your tap TDS above 300–400 ppm? RO is helpful.
• Do you have chloramine? Prefer RO with good carbon blocks (and DI for reef).
• Are you fighting algae fed by phosphate or silicate? RO/DI helps.
• Are your fish/shrimp/frogs from soft-water habitats? RO plus remineralization is ideal.

Understanding Key Numbers: TDS, GH, KH, and pH

TDS (Total Dissolved Solids)

TDS is a rough measure (in ppm) of all dissolved ions. RO typically brings TDS below 10–30 ppm; RODI can reach 0–5 ppm. TDS doesn’t tell you which ions are present, only how much total. Use it for quick checks and membrane health.

GH, KH, and pH in plain language

• GH (general hardness) measures calcium and magnesium. 1 dGH ≈ 17.9 mg/L as CaCO3.
• KH (carbonate hardness/alkalinity) measures buffering against pH swings. 1 dKH ≈ 17.9 mg/L as CaCO3.
• pH tells you how acidic/basic water is. With very low KH (like pure RO), pH can swing easily, so you must add buffer for stability unless you’re doing a specialized blackwater setup.

Target ranges for common setups

• General community: GH 4–8 dGH, KH 3–6 dKH, pH around 6.8–7.4
• Soft-water species (discus/apistos): GH 1–4, KH 0–2, pH 5.5–6.8 (stable)
• Neocaridina shrimp: GH 6–8, KH 3–6, pH 6.8–7.5
• Caridina shrimp (crystal/bee): GH 4–6, KH 0–1, pH 5.8–6.6
• Reef: Use RODI to 0–5 TDS, then mix with reef salt to desired salinity (e.g., 1.025 specific gravity), then manage calcium, alkalinity, and magnesium within reef targets.

Building or Choosing an RO System

Core parts of an RO unit

• Sediment filter (usually 5–1 micron): catches rust, sand, fine particles.
• Carbon block(s): remove chlorine and chloramine that can damage the membrane.
• RO membrane: the heart of the system (common sizes: 50, 75, 100 GPD).
• Optional DI resin cartridge: polishes remaining ions for near-zero TDS.
• Flow restrictor and waste line: maintain proper pressure across membrane.
• Auto shutoff valve (ASOV) and check valve: stop flow when storage container is full.
• Inline TDS meters (optional but helpful): monitor before/after DI.

Membrane size and rejection rate

GPD (gallons per day) ratings assume ideal conditions (77°F/25°C and 50–60 psi). Real production is often lower. Look for a membrane with 96–99% rejection for consistent low TDS. The 75 GPD FilmTec-style membranes are popular for their balance of efficiency and rejection.

Water pressure and temperature matter

Higher pressure and warmer water increase production and improve rejection. If your home pressure is below ~50 psi, a booster pump helps. Cold water slows output; in winter, expect slower fill times.

Wastewater ratio and how to improve it

Typical RO units send 2–4 gallons of waste for every gallon of RO water. You can reduce waste by using a permeate pump, optimizing pressure, or choosing high-rejection membranes. Never remove the flow restrictor entirely; the membrane needs backpressure to work correctly.

Chlorine vs chloramine

Many cities use chloramine (chlorine + ammonia). Use a high-quality carbon block rated for chloramine. Poor carbon filtration can let chloramine reach the membrane, reducing its life and letting ammonia slip through to the permeate. DI resin removes any remaining ammonia effectively, which is another reason reef keepers favor RODI.

Installing and Using RO Water at Home

Hookup options

• Temporary faucet adapter: Easy to attach and remove from a kitchen or bathroom faucet.
• Garden hose adapter: Works in a laundry room or outside spigot.
• Under-sink install: More permanent, with a dedicated drinking/RO faucet and a storage tank for household use.

First-time setup and flushing

Always flush new carbon blocks until the water runs clear (carbon dust is normal). Then flush the new RO membrane as instructed (often 30–60 minutes) to remove preservatives. Discard this initial water.

Safe storage

• Use food-grade containers (HDPE barrels, water jugs) with lids.
• Keep out of sunlight to prevent algae growth. Dark containers are best.
• Rinse with a little RO water before each refill; periodically sanitize with diluted unscented bleach (then rinse thoroughly).
• Avoid long stagnation. Use stored RO within 1–2 weeks if possible, or circulate with a small pump.

Remineralizing RO Water for Aquariums

Why you must remineralize (freshwater)

Pure RO has almost no GH or KH. That means fish lack needed electrolytes and the pH can swing unpredictably. Remineralization adds essential calcium, magnesium, and buffering so fish, shrimp, plants, and biofilters thrive.

Remineralization methods

• Commercial blends: Easy and consistent. Options include shrimp-specific GH-only salts (for Caridina), all-in-one GH/KH salts for community tanks, and plant-focused mineral mixes (e.g., Seachem Equilibrium plus a KH buffer).
• Separate salts: Using calcium chloride, magnesium sulfate (Epsom salt), potassium bicarbonate, etc. This offers flexibility but requires accurate scales and testing.
• For reefs: Do not add freshwater minerals. Simply mix high-quality reef salt with RODI to the desired salinity. For freshwater top-off in reef tanks, use plain RODI (no salt) so salinity stays stable.

Starter targets and example recipes (freshwater)

Always test and adjust slowly. Use a small digital scale and mix salts in a separate bucket. The numbers below are starting points; brands differ in strength, so check the label.

• General community tank (planted or non-planted): Target GH 4–6 dGH, KH 2–4 dKH, pH ~6.8–7.2. Example using common products per 20 liters (5.3 gallons):
  • Seachem Equilibrium: around 3.5–4.0 g to reach ~5 dGH (adjust to your test results).
  • Potassium bicarbonate or a commercial alkalinity buffer: add slowly to reach ~3 dKH (often ~0.6–0.8 g potassium bicarbonate per 20 L, test to confirm).
• Neocaridina shrimp: GH 6–8, KH 3–6, pH 6.8–7.5. Use a shrimp GH/KH remineralizer per label to the target values, or combine a GH additive (for Ca/Mg) with a KH buffer.
• Caridina shrimp (crystal/bee): GH 4–6, KH 0–1, pH 5.8–6.6. Use a GH-only shrimp mineral (often called “GH+”) to raise GH without boosting KH. Keep KH near zero; pH will sit lower and stable on an active shrimp substrate.
• Discus/soft-water tetras: GH 1–4, KH 0–2, pH 5.5–6.8. Use GH-only minerals sparingly; many keepers run very low KH with careful monitoring for stability.

Mixing steps that prevent surprises

• Measure RO volume in a clean bucket.
• Add minerals a little at a time with a small powerhead or vigorous stirring.
• Test GH and KH after fully dissolved, wait 5–10 minutes, test again.
• Record the grams per bucket so you can repeat the same profile every time.
• Acclimate livestock slowly when switching from tap to RO-based water.

Common Problems and Simple Fixes

“My RO TDS is higher than expected”

• New membrane? It may need an extended flush; TDS often drops after the first few gallons.
• Worn carbon blocks? Chloramine can degrade the membrane, lowering rejection. Replace prefilters on schedule.
• Low pressure or cold water? Both reduce membrane performance. Add a booster pump or warm the feed slightly (never hot).
• High TDS creep at startup? Discard the first minute or two of water, or use a flush valve. An auto-flush kit helps.
• Consider a DI stage to polish the remaining TDS, especially for reef tanks.

“My RO unit produces water too slowly”

• Check pressure: Below ~50 psi? A booster pump increases output.
• Check temperature: Very cold feed water drastically reduces flow.
• Clogged sediment/carbon filters? Replace if pressure drop is high.
• Membrane fouling from hard water scale? Install a better sediment filter, ensure proper waste ratio, and flush regularly.

“I still get algae/diatoms in my tank”

• Silicate can slip through tired membranes or carbon. DI resin removes silicate effectively.
• Check your salt mix (for reef) and feeding habits; RO isn’t a cure-all if nutrients are added elsewhere.

Strange taste, odor, or film in storage

• Sanitize storage barrels and tubing every few months.
• Keep containers covered and out of light.
• Don’t let water sit stagnant for long periods; circulate occasionally.

Maintenance Schedule and Operating Costs

Typical replacement intervals

• Sediment filter: every 6 months (or when pressure drops noticeably).
• Carbon block(s): every 6–12 months, sooner if chloramine is high.
• RO membrane: 2–5 years, depending on use, water quality, and care.
• DI resin: replace when post-DI TDS rises above 1–5 ppm (reef keepers aim for 0).

Sanitizing and storing the system

At least annually, sanitize housings with diluted unscented bleach, rinse thoroughly, and replace filters. If you store the RO unit for a long time, follow manufacturer instructions for membrane preservation. Never let the membrane dry out in normal use.

Cost and practical tips

• Initial unit cost varies; hobbyist-grade RO/DI systems are affordable and often pay for themselves compared to buying water.
• Wastewater can be reused for plants, flushing toilets, mopping floors, or laundry, as long as local regulations allow and the water is suitable for that purpose.
• Using quality prefilters protects the membrane, saving money long-term.

Environmental Considerations

What about the waste line?

RO does produce a brine line. Think of it as concentrated tap water, not toxic waste. You can route it to a garden, trees, or cleaning tasks to reduce environmental impact. Improve efficiency with proper pressure, a permeate pump, and good membranes. For small aquariums, total waste volume is generally modest if you plan your production days and storage well.

Myths and Misconceptions About RO Water

“RO water strips minerals from fish or people”

Fish and humans get minerals from food, not just water. RO isn’t corrosive by itself; it’s simply low in dissolved solids. For freshwater aquariums, you must remineralize to provide the correct ions and buffering. For drinking, many people prefer RO with a remineralization cartridge for taste; that’s a separate household choice.

“RO pH is wrong for my fish”

Pure RO pH is unstable due to near-zero KH, and a meter reading can drift because RO lacks buffering. What matters is the final remineralized values in the aquarium. Adjust GH and KH to your target, and the pH will settle where it should. Stability beats chasing a specific pH number.

“Zero TDS means sterile water”

Not true. DI removes ions, not organisms. Keep storage clean and avoid contamination. In the aquarium, your biological filter and good maintenance provide stability, not “sterility.”

Quick Start Checklist

• Test your tap: TDS, GH, KH, nitrate, and, if possible, check for chloramine.
• Choose an RO or RO/DI unit sized for your needs; include quality sediment and carbon filters.
• Install with proper pressure; consider a booster pump if below ~50 psi.
• Flush new filters and membrane before collecting water.
• Store RO in clean, food-grade, covered containers out of sunlight.
• Remineralize RO for freshwater using a trusted product; test GH and KH and record your recipe.
• For reef: use RODI and mix with reef salt to target salinity; top off evaporation with plain RODI.
• Maintain filters on schedule and monitor TDS before and after DI.

Examples: Matching RO Water to Popular Aquarium Types

Low-tech planted community tank

Start with RO, add a balanced remineralizer to GH 5 and KH 3, aim for pH around 7. Keep nitrate and phosphate in check with regular water changes. Most common community fish will thrive and plants will have the calcium and magnesium they need.

Discus display

Use RO, raise GH to 2–3, keep KH 0–1 for a gentle, stable pH near 6–6.5. Prioritize stability and cleanliness. Feed carefully and do frequent water changes with water matched in temperature and parameters.

Caridina shrimp

RO plus a GH-only shrimp mineral to GH 5–6, KH near zero, pH near 6–6.4 with an active substrate. Slow drip acclimation on any parameter change. Consistency is key.

Reef tank

RODI to 0 TDS, mix high-quality reef salt to 1.025 specific gravity (35 ppt). Maintain calcium, alkalinity, and magnesium with testing and dosing. For evaporation, top off only with pure RODI to maintain salinity.

Troubleshooting Chloramine and Ammonia

Understanding the chemistry

Carbon blocks break chloramine into chlorine and ammonia. The membrane and DI then remove these. If your post-membrane TDS is higher than expected, or you detect ammonia/chlorine after the carbon, it’s time for fresh carbon blocks or an extra carbon stage. Inline chlorine/chloramine test strips can confirm performance.

Frequently Asked Practical Questions

How long can I store RO water?

In a clean, covered, food-grade container out of light, 1–2 weeks is a safe guideline. If it sits longer, circulate and consider sanitizing the container periodically.

Can I remineralize directly in the aquarium?

Better to mix in a separate bucket or barrel so you can test GH and KH before adding to the tank. This avoids accidental spikes.

Do I need to remineralize top-off water?

• Freshwater: If your tank evaporates, only water leaves; minerals stay. Top off with RO to keep parameters stable. Remineralize during water changes, not during small daily top-offs.
• Reef: Top off with plain RODI. Salinity rises if you add saltwater for top-off.

Conclusion

RO water isn’t mysterious—it’s simply a clean slate. By removing most dissolved substances, reverse osmosis lets you build the exact water profile your fish, shrimp, plants, and corals prefer. For freshwater, that means adding back the right minerals for GH and KH so pH stays steady and livestock stays healthy. For reef systems, RODI gives you a consistent base for mixing salt and preventing nuisance nutrients.

If your tap water is hard, inconsistent, or treated with chloramine, an RO or RO/DI unit can make your aquarium life easier and your results more predictable. Choose a quality unit, maintain prefilters and membrane, store water properly, and keep simple records of your remineralization recipe. With these habits, you’ll have stable parameters, fewer algae headaches, and happier aquatic life. That’s the real power of understanding what RO water is and how to use it well.