RO/DI — the foundation of clean water

Everything added to the tank begins with water. If the foundation is impure, every water change and every litre of top-off carries problems that no other method can remove.

An RO/DI unit is the reef aquarist’s single most important purchase — not the rarest coral, not expensive lighting, not the most sophisticated dosing system. Everything else is secondary if the source water does not meet basic quality requirements. This is not opinion. It follows from how precise reef water chemistry is: when measuring micrograms per litre, tap water background levels can already be multiples of the target.

1. What Finnish tap water contains

Finnish drinking water is exceptionally pure by world standards. Finland meets the EU Drinking Water Directive requirements, monitoring is rigorous, and tap water is safe to drink virtually everywhere.

This purity is drinking water purity — not reef water purity. These are different standards.

Typical tap water parameters in Finland:

• pH: 6.5–8.5 (varies considerably; in the Helsinki region raised to 7.5–8.5 by lime treatment)
• Hardness: soft almost everywhere; Helsinki region 2.7–4.5 °dH
• Nitrate: typically below 5 mg/l, but in well water and agricultural areas can reach 10–20 mg/l
• Silicates: typically 2–15 mg/l in Finnish groundwater
• Copper: national limit 2 mg/l, but in old copper pipework can leach at elevated concentrations from cold standing water
• Chlorine or chloramine: used for disinfection; chloramine has been in use in the Helsinki region

Reef water targets are a different matter. The target for silicates is effectively zero. Copper is toxic to invertebrates at µg/l levels. Nitrate at 5 mg/l is five times the reef target. Chloramine does not come out with ordinary carbon filtration at all.

2. Why “zero TDS” does not mean clean water

TDS (Total Dissolved Solids) is an electrical conductivity measurement. The device measures ions’ ability to conduct electricity in water.

TDS measures ions, not organic compounds. Many harmful substances — pesticides, pharmaceutical residues, organic pollutants — are electrically neutral molecules. They do not show up in a TDS meter at all.

A TDS meter cannot assess concentrations of individual ions. The meter gives the sum of all ions — not whether the water contains too much silicate, sodium or nitrate.

Practical significance: TDS is an essential diagnostic tool for monitoring RO/DI unit function, but it is insufficient as a general measure of water purity.

3. How an RO/DI unit works

An RO/DI unit has two separate purification stages with different operating principles and different target compounds.

Reverse osmosis membrane (RO)

A reverse osmosis membrane is a thin-film filter with a pore size of about 0.0001 micrometres — smaller than individual ions. Pressure forces water through the membrane, and most dissolved substances remain in the reject stream that is sent to drain.

An RO membrane typically removes:

• 95–99% of dissolved salts (Ca, Mg, Na, K, Cl, SO₄)
• 95–99% of nitrate, phosphate, silicate
• Most heavy metals (Cu, Pb, Zn, Hg)
• Most microbial contamination

Chloramine in the RO filter: Chloramine molecules are small enough to pass through the RO membrane, and ordinary granular activated carbon does not remove chloramine effectively due to insufficient contact time. Chloramine requires its own filtration media (catalytic activated carbon or CAB media), and if source water analysis reveals chloramine use, dedicated removal must be in place before the RO membrane.

Deioniser (DI)

The deioniser contains ion-exchange resin — typically a mixed-bed cartridge with both cation-exchange (H⁺) and anion-exchange resin (OH⁻). The resin replaces all remaining ions with water molecules: cations exchanged for H⁺ and anions for OH⁻, which combine to form pure water.

DI removes practically all remaining ions. DI cartridges are consumable: when the resin is exhausted, its separation capacity collapses abruptly. This shows in the TDS meter.

DI exhaustion does not always give clear warning. Resin capacity remains good almost to the end, then collapses. A long-used, failed DI cartridge passes tap water nitrate, silicate and other ions completely unfiltered — and this can be a hidden source of surprising nutrient load.

4. Chloramine — an important note for Finnish hobbyists

Chloramine (NH₂Cl) forms when water utilities add both chlorine and ammonia. The combination is more stable than plain chlorine and remains effective over long distribution networks. It is used particularly in large cities where water travels long distances before reaching the tap. The Helsinki region has used chloramine.

Chloramine is a triple problem in a reef aquarium:

It does not come out with ordinary activated carbon. Granular activated carbon removes chlorine effectively, but chloramine requires either catalytic activated carbon (CAGC) or a dedicated chloramine filter.

It can pass through the RO membrane. Chloramine molecules are small enough to bypass reverse osmosis. This means chloramine must be removed before the RO membrane, not after.

Chloramine breakdown releases ammonia. If chloramine enters the tank, it slowly breaks down and releases ammonia.

Finnish hobbyists should check their water utility’s disinfection practice. The information is usually available in the utility’s annual report or by calling directly.

5. Silicate — the quiet algae seed

Silicate (SiO₂) is one of the most common tap water impurities in a reef aquarium. Finnish groundwater typically contains 2–15 mg/l silicate.

Diatoms (Bacillariophyta) use silicate to build their frustules. In new aquariums, diatoms are almost invariably the first algae problem: a brown or golden-brown coating on rocks, substrate and glass. This diatom bloom is entirely normal while silicate is available and disappears on its own once silicate reserves are depleted. If source water continuously brings more silicate, the diatom bloom never ends.

RO/DI removes silicate effectively: the RO membrane removes over 95% and DI resins finish the job.

6. Copper and heavy metals

Copper is among the most toxic common metals for invertebrates. The national drinking water limit is 2 mg/l. The reef aquarium target is below 5 µg/l — several hundred times lower.

What makes tap water copper particularly insidious is that concentrations vary with use. Cold water that has stood in copper pipework overnight can contain significantly higher copper levels than water running during the day.

Practical tip: run the tap for a moment before starting your RO/DI unit in the morning, particularly if you have old copper plumbing.

7. A complete RO/DI system and cartridge monitoring

A functional RO/DI system for basic aquarium use typically consists of:

1. Sediment filter (5 µm): mechanical pre-filtration
2. Activated carbon filter: removes chlorine to protect the RO membrane; chloramine removal requires catalytic media
3. RO membrane: the main purification stage; removes 95–99% of dissolved substances
4. DI cartridge: the finishing stage; TDS on the output side must be 0 ppm

Practical advice: install two TDS meters — one before the RO membrane (feed water) and one after the DI cartridge (output water). The ratio tells you RO membrane efficiency, and the final reading tells you DI condition.

8. Three common mistakes

Using only an RO unit without a DI stage

A plain RO unit without a DI stage does not produce reef-quality water. The RO membrane removes 95–99% of salts — but that 1–5% remaining is significant in reef chemistry. A half-system is misleading: TDS may read 5–20 ppm, which seems small, but means a continuous flow of silicate, nitrate and other ions into the tank with every water change.

Not replacing cartridges

RO/DI unit filters are not lifetime components. A unit that has been purchased but whose cartridges have been left unchanged is worse than no unit at all — because it creates a false sense of security without the protective function.

Unit has been sitting unused for a long time

Cartridges age in storage. Activated carbon absorbs moisture and can grow bacteria, DI resin oxidises over time. The RO membrane dries out during prolonged storage and its performance degrades.

Practical guideline: if the unit has been unused for more than 3 months, replace all cartridges before use.

Summary

RO/DI is the reef aquarist’s only quality source water solution. Finnish tap water is excellent drinking water, but reef water standards are an order of magnitude stricter than drinking water standards. Silicates, chloramine, copper, nitrate and organic impurities are all present in tap water at levels that are meaningful for reef biology.

Zero TDS does not mean clean water — it means ion-free. Monitor two TDS readings, replace the DI cartridge when the reading rises, and check your utility’s disinfection practice for chloramine.

References

1. Peer-reviewed studies

• Nissinen, T. K. et al. (2018). The Seasonality of Nitrite Concentrations in a Chloraminated Drinking Water Distribution System. Water Research.

2. Hobbyist literature and brand documentation

• Holmes-Farley, R. (2004). Aquarium Chemistry: Tap Water in Reef Aquaria. Reefs.com. https://reefs.com/magazine/aquarium-chemistry-tap-water-in-reef-aquaria/
• HSY (2024). Water quality in the Helsinki Metropolitan Area. https://www.hsy.fi/vesi-ja-viemarit/veden-laatu/

3. Books and textbooks

• Borneman, E. H. (2001). Aquarium Corals: Selection, Husbandry, and Natural History. TFH Publications.
• Calfo, A. & Fenner, R. (2003). Reef Invertebrates: An Essential Guide to Selection, Care and Compatibility. Reading Trees / Wet Web Media.