RecifAtHome guide
Water is the foundation of a reef aquarium. A reverse osmosis unit produces purified water free from the nitrate, phosphate, silicate and metals found in tap water: no other piece of equipment does more to prevent algae problems.
In short
- Tap water contains substances that are undesirable in a reef tank.
- The goal is water at 0 to 1 ppm TDS after the DI resin.
- Replace pre-filters every 6 months and the membrane every 2 to 3 years.
- RO water is used to mix saltwater and top up evaporation losses.
Why not use tap water?
Tap water is safe to drink but unsuitable for a reef tank: it may contain nitrate, phosphate, silicate (which fuels diatoms), chlorine or chloramine, and traces of copper from pipework. These substances encourage algae and harm corals and invertebrates.
How does a reverse osmosis unit work?
A reverse osmosis unit uses mains water pressure to push water through several filtration stages:
- A sediment pre-filter (5 microns) traps particles.
- One or two carbon filters remove chlorine, which would damage the membrane.
- The reverse osmosis membrane rejects more than 95 % of dissolved salts.
- A deionisation resin removes the remaining traces and brings the water down to 0 ppm.
Some water flushes the membrane and goes down the drain: depending on the model, 2 to 4 litres are used to produce 1 litre of purified water. This waste water can be used for watering plants.
What capacity should you choose?
RO capacity is expressed in GPD (gallons per day). For a 100 to 300-litre aquarium, a 50 to 100 GPD unit (190 to 380 litres per day in ideal conditions) is enough. Actual output depends on water pressure and temperature: below 3 bar, a booster pump significantly improves output.
Maintenance
| Component | Replacement interval |
|---|---|
| Sediment pre-filter | 6 months |
| Carbon | 6 months |
| Membrane | 2 to 3 years |
| DI resin | As soon as TDS exceeds 1 ppm |
A TDS monitor on the outlet shows when the resin needs replacing. Cartridges and refills can be changed without tools on most models.
Using RO water correctly
- Mix saltwater in a separate container with a pump and heater a few hours before the water change, following the salt manufacturer's instructions.
- Replace evaporation with RO water only: only water evaporates, the salt stays. An auto top-off does this automatically.
- Store RO water in a clean, closed container.
Frequently asked questions
Can I use tap water in a saltwater aquarium?
It is not recommended: tap water introduces nitrate, phosphate, silicate and sometimes copper, which encourage algae and harm corals.
What TDS should I aim for?
0 ppm after the DI resin. Above 1 ppm, it is time to replace it.
How much waste water does an RO unit produce?
Depending on the model, 2 to 4 litres of waste water per litre of purified water. A booster pump or a new membrane improves this ratio.
When should I replace the membrane?
Usually every 2 to 3 years, or when the TDS measured right after the membrane rises noticeably.
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Guide written by the RecifAtHome team, coral aquaculture in Roche-lez-Beaupré (France) since 2010. Updated: October 2026.