In saltwater aquarium keeping, "water preparation" is the foundation of successful husbandry. While freshwater tanks can be set up simply by dechlorinating tap water, saltwater tanks often require much more. Impurities found in tap water—such as silicates, phosphates, nitrates, and heavy metals—can lead to algae outbreaks and coral decline.
The proper approach is to create pure water using an RO (Reverse Osmosis) water purifier or DI (Deionized) filter, then dissolve high-quality artificial sea salt into that water to prepare your aquarium water. This article explains everything you need to know: how RO/DI purifiers work, how to choose one, how to properly dissolve artificial sea salt, and storage methods.
What Are RO/DI Water Purifiers?
RO (Reverse Osmosis) Filter
An RO membrane is an ultrafine semipermeable membrane that allows only water molecules to pass through. By forcing tap water through the membrane under high pressure, you can remove 90–99% of dissolved impurities. These include chlorine, heavy metals, silicates, phosphates, and nitrates—essentially eliminating components harmful to aquariums.
DI (Deionized) Filter
A DI filter is a polishing stage that removes the small amount of ions remaining after RO filtration. DI resin captures trace impurities that the RO membrane cannot remove completely, bringing TDS (Total Dissolved Solids) close to 0 ppm.
RO Alone vs. RO/DI
| Specification | RO Only | RO/DI |
|---|
| Impurity removal rate | 90–98% | 99%+ |
| Output water TDS | 5–20 ppm | 0–2 ppm |
| Best for | Fish-only tanks | All coral tank types |
| Running cost | Slightly lower | DI resin replacement adds cost |
For fish-only tanks, RO purification alone is sufficient. However, for coral keeping, adding a DI stage (RO/DI system) is highly recommended.
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How to Choose an RO/DI Water Purifier
Production Capacity (GPD)
RO purifier capacity is measured in GPD (Gallons Per Day). Hobby-grade saltwater aquarium models typically range from 50 to 200 GPD.
| Tank Size | Recommended GPD |
|---|
| Up to 60 cm (60 L or less) | 50–75 GPD |
| 60–90 cm (60–200 L) | 75–100 GPD |
| 90–120 cm (200–400 L) | 100–150 GPD |
| 120 cm+ (400 L+) | 150–200 GPD |
Higher GPD produces more pure water faster, but system size and cost increase accordingly. A good rule of thumb is to choose a GPD that can produce your weekly water change requirement within half a day.
Number of Filter Stages
A typical RO/DI system has 4–5 stages:
- Sediment Filter (5μm): Removes sand, rust, and large particles
- Carbon Block Filter: Removes chlorine, chloramine, and organic compounds
- Carbon Block Filter (2nd stage): Ensures complete chlorine removal. Since RO membranes are chlorine-sensitive, this stage is critical for membrane lifespan
- RO Membrane: Removes 90–99% of impurities
- DI Resin: Adsorbs residual ions and brings TDS to 0 ppm
More stages mean higher purification quality and longer-lasting membranes and resin.
TDS Meter Is Essential
If you use an RO/DI system, a TDS (Total Dissolved Solids) meter is a must-have. Inline TDS meters placed before and after each stage help you determine exactly when to replace filters.
- Before RO membrane: Check tap water TDS (typically 30–300+ ppm depending on location)
- After RO membrane: Verify removal rate (should be 2–10% of original tap water TDS)
- After DI resin: Ideal is 0 ppm. Replace DI resin when readings reach 1 ppm or above
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Filter and Membrane Replacement Schedule
Every component in an RO/DI system has a finite lifespan and requires periodic replacement. Neglecting replacements reduces purification efficiency and allows impurities back into your aquarium.
| Component | Replacement Interval |
|---|
| Sediment filter | 6–12 months |
| Carbon block filter | 6–12 months |
| RO membrane | 2–3 years |
| DI resin | When color changes (replace when resin discolors) |
These timelines are approximate and vary based on tap water TDS and usage volume. Monitor your TDS meter regularly and replace components as soon as efficiency drops.
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Choosing Artificial Sea Salt
Once you have RO/DI water, the next step is choosing artificial sea salt. There are two main types:
Standard Type
Formulated to closely match natural seawater composition. Suitable for fish-centric systems and soft corals or less demanding coral species. Cost-effective with good value for money.
Premium Type (Reef Salt)
Enriched with extra calcium, magnesium, and trace elements compared to natural seawater. Ideal for SPS coral keeping and color enhancement. Slightly more expensive, though it may reduce the need for additional supplements.
Characteristics of Major Sea Salt Brands
Popular artificial sea salt brands available domestically include:
- Red Sea Salt: Offers both standard and "Coral Pro" lines. Well-regarded for consistent composition
- Instant Ocean: A classic budget-friendly option. Dissolves quickly and is easy to use
- Tropic Marin: German-made premium salt. "Pro Reef" is popular among SPS keepers
- Live Sea Salt: A domestic brand designed with Japanese tap water conditions in mind
Once you choose a brand, stick with it. Frequently switching brands can cause subtle fluctuations in water parameters and stress your livestock.
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Proper Dissolving Method for Artificial Sea Salt
How you dissolve artificial sea salt affects your final water quality. Follow these steps for stable, reliable aquarium water:
Step 1: Collect RO/DI Water in a Tank
Pour the needed amount of RO/DI water into a bucket, plastic container, or dedicated holding tank. For water changes, aim for 10–15% of your tank volume.
Step 2: Adjust Water Temperature
Use a heater to bring the water to 24–26°C (the same as your display tank). Cold water dissolves salt slowly, and adding temperature-mismatched water stresses your livestock.
Step 3: Add Artificial Sea Salt Gradually
With a circulation pump or airstone running, add salt powder slowly in small portions. Adding too much at once causes settling and undissolved residue. Follow the manufacturer's guidelines while checking specific gravity as you go.
Step 4: Verify and Adjust Specific Gravity
Use a hydrometer (refractometer or floating hydrometer) to check specific gravity. Standard saltwater aquariums target 1.023–1.025 (approximately 35 ppt).
- Refractometer: More accurate; recommended. Requires only a few drops
- Floating Hydrometer: Affordable but less reliable due to air bubble attachment and age-related drift
If gravity is too high, add more RO/DI water. If too low, add a bit more salt.
Step 5: Aerate and Age the Water for Several Hours
Freshly dissolved salt water often has unstable pH and KH (carbonate hardness). Run aeration and circulation for 4–24 hours before use.
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Storing Prepared Seawater
If you prepare seawater in advance, follow these guidelines:
- Use sealed containers: Open storage allows evaporation (raising specific gravity), contamination, and bacterial growth
- Store in darkness: Light promotes algae and other growth
- Install aeration or circulation: Maintains dissolved oxygen and prevents sediment buildup
- Use within one week: Water quality degrades over time; make only what you need
RO/DI pure water (before adding salt) can be stored in sealed containers for several weeks, but it absorbs CO2 from air and becomes acidic, so use it promptly.
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Why Avoid Using Tap Water Directly
You might wonder: "Isn't dechlorination enough?" While dechlorination removes chlorine and chloramine, it leaves these substances behind:
- Silicates: The primary nutrient for diatom (brown algae). High silicate levels in tap water mean endless diatom blooms
- Phosphates: A cause of green algae and cyanobacteria. Also inhibits coral calcification
- Nitrates: Present in tap water in some regions. You want to lower nitrates through water changes, not add more
- Heavy metals (copper, zinc, etc.): Can leach from old pipes and harm invertebrates and corals
Fish-only tanks can survive on dechlorinated tap water, but for coral keeping or maintaining crystal-clear, low-algae tanks, an RO/DI purifier is essentially mandatory.
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Tips for Installing RO/DI Purifiers
Connecting to Your Water Supply
Most RO/DI systems connect to a faucet adapter. You can also use a branch connector on a washing machine valve or install a saddle valve on the supply line under your sink.
Dealing with Wastewater (Concentrate)
RO purifiers produce wastewater alongside pure water—typically 2–4 liters of waste per 1 liter of pure water. Reuse this wastewater for plants, laundry, or cleaning to minimize waste.
Water Pressure Requirements
RO membranes need 40–80 PSI (approximately 2.8–5.5 kg/cm²) to function efficiently. Most Japanese homes fall within this range, but high-rise apartments or older buildings may have lower pressure. If needed, add a booster pump to solve the problem.
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Get Expert Advice from Brichoku
Water preparation is the foundation of saltwater aquarium success. When it's stable, livestock thrive. However, personal questions like "What RO grade do I need for my tap water's TDS?" or "Which sea salt brand is right for me?" are hard to answer from general information alone.
On Brichoku, you can buy directly from professional saltwater fish and coral breeders and ask them about the RO/DI purifiers they actually use, which sea salt brands work best, and professional water-management tips. Even if water preparation feels daunting, you can start your saltwater aquarium with confidence thanks to breeder support. Find your ideal livestock on Brichoku and create the perfect environment for your animals.
