How the four stages work together
Most aquarium RODI units run sediment → carbon → RO membrane → DI. Three-stage units stop after the membrane, so you’ll need an extra DI pod or cartridge if you want 0 ppm TDS water. Some household units use different cartridge shapes, so match replacements to your housing and model rather than buying what looks the same.
| Stage | What it does | When to replace it | Current UK example |
|---|---|---|---|
| Sediment filter | Catches particles before they reach the membrane | It’s visibly dirty, pressure has dropped or the maker’s interval is up | £3.99 for a D-D 10-inch |
| Carbon block | Removes chlorine and protects the membrane | On a set schedule, before chlorine gets through. Don’t wait for problems in the tank | £6.99 for a D-D 10-inch |
| RO membrane | Removes most dissolved solids before the DI | When rejection (tap TDS vs post-RO TDS) falls | £48.99–£56.99 for a TMC Pure 100 |
| DI resin / pod | Removes what’s left, down to 0 ppm | When the reading after the DI rises above 0, or the resin is spent | £19.20 for a TMC 500 ml pod |
Is your membrane still working?
Take a TDS reading of your tap water and another straight after the membrane, before the DI. Rejection is ((tap TDS − post-RO TDS) ÷ tap TDS) × 100. For example, 280 ppm from the tap and 11 ppm after the membrane is about 96.1% rejection. What counts as acceptable depends on the membrane and conditions, so watch your own trend rather than comparing one reading with a number from a forum.
What will it cost you each year?
As an example, one sediment cartridge (£3.99), one carbon block (£6.99) and two £19.20 DI pods come to about £49.38, before the membrane and your water bill. Add a £56.99 membrane every 2–3 years and you’re looking at roughly £68–£77 a year in parts. That’s an example for planning, not a promise: high TDS, chloramine, silica or heavy use can use up resin much faster.
Don’t forget the waste water
Conventional units typically send 3–4 litres to the drain for every litre of pure water. So making 100 litres of RODI water can mean roughly 300–400 litres of waste. You can collect it for suitable uses outside the aquarium, but never put it back in your reef or use it anywhere the extra contaminants could cause harm.
Sources and limits
We checked these examples against current UK listings for D-D sediment cartridges, D-D carbon cartridges, TMC membranes, TMC replacement parts and Osmotics filter kits. We haven’t tested how long cartridges last on any particular UK water supply.
THE SHORT ANSWER
Replace each stage when the readings say so, but never let carbon run out
The sediment filter catches particles before they reach the later stages. Carbon protects the membrane from chlorine, chloramine and organics. The RO membrane removes most dissolved solids, and the DI resin polishes out what’s left. Each stage wears out differently, so one calendar date can’t tell you what needs changing.
Keep track of your water pressure, tap TDS, the TDS after the membrane and the TDS of the final water. A pressure drop or a visibly dirty cartridge points to the sediment filter. The maker’s rated capacity and your local disinfectant decide when carbon goes. Rejection percentage tells you about the membrane, and rising final TDS means the DI resin is spent. Change carbon before it runs out, because waiting until the membrane is damaged is too late.
STEP BY STEP
How to do it
- 01
Take baseline readings
While everything’s working well, note the pressure with clean filters, your tap TDS, the TDS after the membrane, the final TDS and roughly how fast it makes water.
- 02
Find the stage that’s struggling
A pressure drop suggests a clogged pre-filter. Worse rejection points to the membrane or its conditions. Rising final TDS means the DI resin is used up or water is channelling through it.
- 03
Check the conditions first
Temperature and pressure affect both output and rejection. Rule those out before you replace a membrane that may be fine.
- 04
Log every replacement and its cost
Note the date, roughly how many litres it made, the part number and the readings before and after. Next time, you’ll replace on evidence rather than habit.
YOUR SITUATION
What to do in your situation
The reading after the pre-filter falls and output slows
Check the sediment filter, and replace it if it’s clogged.
Rejection is worse in similar conditions
Check pressure, temperature, the flow restrictor and the membrane itself.
The water after the DI no longer reads the zero you aim for
Replace or refill the DI resin, and check water flows evenly through it.
COMMON MISTAKES
Mistakes to avoid
Trusting the resin colour
The colour change can be patchy or lag behind. Confirm with a reliable TDS reading.
Running carbon past its limit
Chlorine or chloramine can damage the membrane long before your readings make the cause obvious.
Changing everything at once
Replacing every stage together hides which one actually failed, and what the unit really costs to run.
WORKED EXAMPLE
Telling a membrane problem from a DI cost
Tap water reads 280 ppm TDS and water after the membrane reads 8 ppm, in steady conditions. That’s about 97.1% rejection, so the membrane is doing its job. If the final water starts reading above target, look at the DI resin first, not the membrane.
If the reading after the membrane climbs to 25 ppm instead, the resin will run out much faster, even though it can hide the problem for a while. Check pressure and temperature, then the membrane’s rejection and the flow restrictor, before you keep buying more resin.
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
How often should RODI filters be changed?
Follow the maker’s limits, alongside your pressure, TDS, rejection and local water. No single interval suits every stage.
How do I work out membrane rejection?
Take the TDS after the membrane away from your tap TDS, divide by your tap TDS and multiply by 100, using readings taken in similar conditions.
Does zero TDS prove my unit is healthy?
No. DI resin can mask a weak membrane for a while, so check the water after the membrane too.