Essential guide to RO/DI water purification and synthetic saltwater preparation: system selection, TDS monitoring, filter maintenance, salt mixing procedures, and specific gravity adjustment.
Key Takeaways
Essential guide to RO/DI water purification and synthetic saltwater preparation: system selection, TDS monitoring, filter maintenance, salt mixing procedures, and specific gravity adjustment.
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.
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.
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.
| 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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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.
A typical RO/DI system has 4–5 stages:
More stages mean higher purification quality and longer-lasting membranes and resin.
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.
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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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Once you have RO/DI water, the next step is choosing artificial sea salt. There are two main types:
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.
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.
Popular artificial sea salt brands available domestically include:
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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How you dissolve artificial sea salt affects your final water quality. Follow these steps for stable, reliable aquarium water:
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.
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.
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.
Use a hydrometer (refractometer or floating hydrometer) to check specific gravity. Standard saltwater aquariums target 1.023–1.025 (approximately 35 ppt).
If gravity is too high, add more RO/DI water. If too low, add a bit more salt.
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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If you prepare seawater in advance, follow these guidelines:
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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You might wonder: "Isn't dechlorination enough?" While dechlorination removes chlorine and chloramine, it leaves these substances behind:
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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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.
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.
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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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.
Find Marine Fish listings related to this article on BreederDirect. Buy directly from verified breeders.
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