RO/DI water is the simplest upgrade for reef stability. It removes unknowns from tap water. That gives you control over nutrients, metals, and algae triggers.
Why RO/DI matters in a reef tank
Tap water varies by season and neighborhood. It can contain nitrate, phosphate, and silicate. Those fuel hair algae and diatoms. It may also carry copper from plumbing.
RO/DI means reverse osmosis plus deionization. RO removes most dissolved solids. DI polishes what remains. Your goal is 0 TDS water for mixing salt.
Good source water supports stable alkalinity and calcium. It also makes dosing predictable. With tap water, buffers can behave differently week to week. That causes swings.
Use RO/DI for top-off and saltwater. Evaporation leaves salts behind. Top-off must be fresh water only. If that water carries nitrate, it builds up fast.
- Target source water: 0 TDS after DI
- Typical reef salinity: 1.025–1.026 at 77°F
- Top-off water: always RO/DI, never saltwater
For setup basics, see reef tank basics. For algae control, read nuisance algae control.
Choosing and setting up an RO/DI unit
Most reefers do well with a 4 to 6 stage unit. A common layout is sediment, carbon, RO membrane, then DI. Add a second carbon block if your city uses chloramine. Chloramine can damage membranes.
Match the unit to your water pressure. Aim for 60 to 80 psi at the membrane. Low pressure reduces production and increases TDS creep. A booster pump helps if you sit under 50 psi.
Install a dual inline TDS meter. Measure before DI and after DI. Also log your tap TDS. Example: tap 250 TDS, post-RO 10 TDS is normal. Post-DI should read 0.
Flush the membrane at start-up and after long pauses. Discard the first few gallons. That reduces initial high TDS water. Store product water in food-safe containers with lids.
- Run water for 5 minutes before collecting product water
- Use a float valve or auto shutoff to prevent floods
- Label containers “RO/DI only” to avoid soap residue
For safe storage and mixing, see mixing saltwater guide.
Maintenance, testing, and troubleshooting
Change sediment and carbon filters on a schedule. Many hobbyists swap them every 6 months. Do it sooner with heavy use. A clogged sediment filter drops pressure fast.
Replace DI resin when the final TDS rises above 0. Do not wait for 2 or 3. Those ions can include silicate and phosphate. Color-changing resin helps, but trust the meter.
Watch membrane performance with rejection rate. Use this math: (tap TDS − post-RO TDS) ÷ tap TDS. Aim for 96% or higher. If it drops, check carbon blocks first. Chlorine can ruin membranes.
Common mistake: collecting water too fast with warm tap. Hot water can damage filters. Use cold water only. Another mistake is letting product water sit with an open lid. Dust can add TDS back.
- If post-DI TDS is 1: replace DI resin and recheck
- If post-RO TDS is high: verify pressure and carbon blocks
- If flow is slow: replace sediment filter and check for kinks
Sources: SpectraPure RO/DI FAQ; Bulk Reef Supply RO/DI education articles; Randy Holmes-Farley, Reefkeeping Magazine water purification articles.
RO/DI water removes the guesswork from reef keeping. It reduces algae fuel and metal risks. With simple testing and regular filter changes, your tank stays more stable.
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