The simple sizing formula
Weekly RODI water = water changes + top-off + a reserve for setup or emergencies. Tank size alone won’t tell you. A covered tank may lose very little to evaporation; a warm, open tank with a sump can lose far more.
Worked example: a 200-litre reef
A 10% weekly water change is 20 litres. Add 5 litres of top-off and you need about 25 litres a week, or 100 litres a month. Even a 50 GPD membrane is rated at around 190 litres per day in ideal conditions, so the question isn’t whether it can make enough. It’s whether your pressure, waste ratio and the time you have make it convenient.
| System size | 10% weekly change | Example top-off | RODI water per week | What it means |
|---|---|---|---|---|
| 100 L | 10 L | 3 L | 13 L | 50 GPD is plenty if your pressure is good |
| 200 L | 20 L | 5 L | 25 L | 50–100 GPD makes it quickly |
| 400 L | 40 L | 10 L | 50 L | 100 GPD makes bigger batches easy |
| 600 L | 60 L | 15 L | 75 L | Look at 100–150 GPD, or make water ahead and store it |
Why “100 GPD” won’t fill your bucket today
100 US gallons per day is about 379 litres per day at the headline rating. That assumes a set pressure and water temperature. Cold UK tap water, low pressure, an old membrane or the wrong flow restrictor can all cut production. Treat the label as a size class, then time how many litres per hour you actually get.
When a booster pump beats a bigger membrane
If your water pressure is below what the unit needs, a bigger membrane won’t fix it. A booster pump can bring the membrane up to its working pressure, and may improve both purity and how much water goes to waste. Check its power supply, automatic shut-off and compatibility before you fit one.
Storage and safety
Most home reefers are best served by a covered container for RODI water and a separate one for mixing salt. Use a float valve or automatic shut-off to cut the risk of an overflow, and never leave a unit running unattended until you’ve tested every connection. Size your storage for one normal water change plus a little extra, not a season’s supply.
Sources and limits
We checked membrane capacities and the pressure caveats against TMC’s Pure 100 information, Aqua Medic’s Professional 100 page and current UK aquarium listings. The top-off figures are examples for planning, not a prediction of your evaporation.
THE SHORT ANSWER
Size it for your weekly use and your real tap water
Add up your water changes, evaporation top-off and an emergency reserve to get your weekly RODI need. Then work out what the membrane will really make at your water pressure and temperature. UK tap water is often colder than the conditions membranes are rated in, so real output can be much slower than the gallons-per-day label suggests.
A bigger membrane only helps if the prefilters, flow restrictor, pressure and DI stage all match it. If your pressure is low, a booster pump may improve both output and purity more than a bigger membrane would. Safe collection and enough storage matter as much as speed.
STEP BY STEP
How to do it
- 01
Work out your weekly water
Add your usual water changes, average top-off and a sensible reserve. Include any quarantine tank or salt mixing that uses the same unit.
- 02
Measure your tap water
Note the pressure, temperature and TDS. Check what disinfectant your water company uses so you have the right carbon filtration.
- 03
Estimate the real production time
Use the maker’s correction figures if they publish them, and assume colder or lower-pressure water will make less. Compare the result with the time you can actually supervise it.
- 04
Plan storage and shut-off
Choose suitable containers, overflow protection and a shut-off you’ve tested. Fast production is no use if collecting it risks a flood.
YOUR SITUATION
What to do in your situation
You need little water each week and can collect it whenever
A modest unit is fine, as long as it has (or can take) a proper DI stage.
Making the weekly litres takes a long time
Go for more capacity, with good pressure and safe storage.
Output is poor and the membrane lets more through
Check what pressure your membrane needs, and consider a matched booster pump.
COMMON MISTAKES
Mistakes to avoid
Taking the GPD figure at face value
The rating comes from test conditions that may not match cold UK water, your pressure or your tap water’s TDS.
Forgetting evaporation
Over time, top-off water can add up to more than your water changes, especially in open, heated tanks.
Leaving it running unattended
A float valve on its own is not a substitute for a tested shut-off and a leak-proof installation.
WORKED EXAMPLE
Sizing for a 250-litre reef
The tank uses 25 litres for a weekly water change and averages 14 litres of top-off, so it needs 39 litres a week as routine. Add a 20-litre reserve and the target is about 59 litres a week. A 100 GPD unit looks like huge overkill on paper, but cold water and low pressure can make production much slower.
If the reefer measures only 6 litres per hour, the weekly target takes nearly ten hours. They could split it over several sessions, add safe storage or boost the pressure. Base the decision on that real timetable, not the 378 litres per day on the label.
BEFORE YOU CHANGE ANYTHING
Write down where you’re starting from
Note the date, the equipment or product you’re using, how you measured and the conditions behind every important number. Say whether a volume is the tank’s label figure or your real net water; whether a flow is the box rating or what actually comes out; and, for test results, the salinity, test kit and time of day. A precise-looking number is useless once you’ve forgotten where it came from.
Change one thing at a time, and decide in advance what improvement will look like: a steady test trend, less detritus, a more stable temperature, a quieter tank or a lower running cost. One reading, or one good day, isn’t a trend, and a setting copied from someone else’s tank isn’t proof it suits yours. The free Reef Companion keeps your tests, maintenance and changes in one dated log, so you can see cause and effect.
MAKING CHANGES SAFELY
Protect your tank while you make the change
Take a baseline
Measure before you install equipment, change your chemistry or alter your routine. Take photos if placement matters, and note how your livestock behaves. Without a starting point, you can’t be sure what caused any difference later.
Start small
Begin with a cautious setting or dose, and keep a way back. Hang on to the old equipment until the new one has proved itself, keep your previous schedule, and don’t add livestock at the same time as a big lighting, flow or chemistry change.
Give it the right amount of time
Mechanical problems show up straight away; nutrient and coral responses can take days or weeks. Judge the change over a sensible period, but stop immediately if you see animals in distress, a leak, overheating or an electrical risk.
Afterwards, note the result and decide whether to keep, adjust or undo the change. If the result is unclear, go back to your last stable setup and measure better, rather than piling on another change.
SIX MONTHS ON
Check it’s still the right decision
What made sense on day one can stop working as livestock grows, the room warms up, equipment gets dirty or a routine turns out to be unrealistic. Set a reminder to review it in six months, comparing what you assumed with the reality: water volume, electricity, consumables, noise, cleaning time and test trends.
Test trends, real flow, temperature, light coverage or whatever the change was for
Is it still doing its job, with room to spare?
How often you clean it, how hard it is to reach, calibration drift and missed tasks
Can you keep this up without cutting corners?
Electricity, reagents, filters, spare parts, warranty and downtime
Would you make the same choice knowing the real cost?
If it’s working, you don’t need to replace it; the review simply gives you a reason to keep it, tune it or plan an upgrade before something fails.
KNOW THE LIMITS
When to get expert help
Get product-specific or professional help for anything involving floor loading, household electrics, unsafe plumbing, damaged glass, unexplained livestock deaths, or chemistry you can’t measure reliably. Use a qualified electrician for fixed wiring, and always follow the current safety instructions for your tank, equipment and additives. Never work around damaged cables, swollen batteries, leaking seals or equipment that keeps tripping the electrics.
Your livestock’s needs can also outweigh general equipment advice. Check adult size, behaviour, disease risk and each species’ light, flow and feeding needs with reputable care guides before you buy. This guide explains the equipment decision; it can’t guarantee the result for every animal or tank.
COMMON QUESTIONS
Frequently asked questions
What size RODI unit do I need?
The smallest unit that can safely make your weekly water in a time you can supervise, at your real pressure and temperature.
Does a bigger membrane waste less water?
Not necessarily. The waste ratio depends on pressure, the flow restrictor, the membrane and how the unit is designed.
Do I need a storage tank?
Not necessarily, but storage makes water changes and emergencies easier. It must be clean, covered and somewhere safe.