The simple sizing formula
Weekly RODI demand = water-change volume + top-off volume + setup or contingency water. Tank volume alone is not enough. A covered tank may evaporate little; an open, warm system with a sump may use much more.
Worked example: a 200-litre reef
A 10% weekly water change is 20 litres. If evaporation and top-off add 5 litres, weekly product demand is about 25 litres, or 100 litres per month. Even a 50 GPD membrane is rated around 190 litres per day under ideal conditions, so the decision is not whether it can make enough over a month—it is whether your pressure, waste ratio and collection window make the process convenient.
| System size | 10% weekly change | Illustrative top-off | Weekly product water | Planning note |
|---|---|---|---|---|
| 100 L | 10 L | 3 L | 13 L | 50 GPD is ample if pressure is healthy |
| 200 L | 20 L | 5 L | 25 L | 50–100 GPD gives plenty of turnaround |
| 400 L | 40 L | 10 L | 50 L | 100 GPD is convenient for larger batches |
| 600 L | 60 L | 15 L | 75 L | Consider 100–150 GPD or a storage-first workflow |
Why “100 GPD” is not 100 gallons in your bucket today
100 US gallons per day is about 379 litres per day at the headline rating. That number assumes stated pressure and temperature. A cold UK feed, low pressure, old membrane or restrictive flow restrictor can reduce production. Treat the label as a capacity class, then measure your actual litres per hour.
When a booster pump makes more sense than a bigger membrane
If your feed pressure is below the system's operating range, a larger membrane does not solve the underlying problem. A booster pump can help the membrane reach its design pressure and may improve rejection and waste behaviour. Check the pump's electrical supply, automatic shut-off and compatibility before fitting it.
Storage and safety
Most home reef keepers benefit from a covered product-water container and a separate salt-mixing container. Use a float valve or automatic shut-off to reduce overflow risk, and never leave a pressurised unit unattended until you have tested every connection. Size the storage container for one normal water change plus a little contingency, not for an unrealistic season's supply.
Sources and limitations
Membrane capacities and pressure caveats were checked against TMC's Pure 100 information, Aqua Medic's professional 100 page and current UK aquarium listings. The worked top-off figures are planning examples, not a prediction of your evaporation rate.