RODI Units: Why UK Tap Water Needs Serious Filtering

Why UK Tap Water Is Bad News for Reef Tanks

UK tap water contains chlorine, chloramine, nitrates (some areas hit 30+ ppm straight from the tap), phosphates, silica, copper, and other heavy metals. Pour that into a reef tank and it's essentially an algae farm with bonus coral death.

What RODI Actually Is

RODI (Reverse Osmosis / Deionisation) is a multi-stage filtration system that strips everything out of tap water, leaving pure H2O:

1. Sediment filter — Catches particles
2. Carbon block — Removes chlorine and chloramine
3. RO membrane — Rejects 95–99% of dissolved solids
4. DI resin — Final polish to 0 TDS

Output should read 0–3 TDS. Above 5, something needs replacing.

Why Not Just Buy RO Water?

Shop RO water costs £1–2 per litre. A 200L tank needs 30–50 litres a week for changes and top-offs. A decent RODI unit costs £80–150 and filters run £50–80 a year — it pays for itself within months.

What to Buy

Budget (£60–100): 4-stage units on Amazon — check reviews carefully for membrane quality Mid-range (£100–200): Reef Pure, Vertex Puratek, Spectrapure — all popular with UK reefers Premium (£200+): High-output units, only needed for very large systems

A 75–100 GPD unit suits most setups.

UK-Specific Note

UK water pressure (40–60 PSI) is often lower than the 65 PSI these units are rated for. A booster pump (~£40–60) helps if pressure is below 50 PSI.

Maintenance

• Sediment & carbon: Every 6 months (~£15–25 total)
• RO membrane: Every 2–3 years (~£30–60)
• DI resin: When it changes colour (~£10–20)

Total annual cost: about £50–80.

Browse used RODI units from UK sellers — the housings last forever, so just replace filters and membrane for essentially a new unit at half price.

What RODI unit is everyone running? Any UK-specific recommendations?