The four-stage logic
Most aquarium RODI systems follow sediment → carbon → RO membrane → DI. Three-stage aquarium units stop after the membrane; they need an additional DI pod or cartridge if your goal is final 0 ppm TDS water. Some domestic systems use different cartridge shapes, so match the part to your housing and model rather than buying by appearance alone.
| Stage | Job | Replacement signal | Current UK example |
|---|---|---|---|
| Sediment filter | Catches particles and protects the membrane | Visible discolouration, pressure drop or the manufacturer's interval | £3.99 D-D 10-inch example |
| Carbon block | Reduces chlorine and protects the RO membrane | Time interval plus chlorine breakthrough risk; do not wait for tank problems | £6.99 D-D 10-inch example |
| RO membrane | Removes most dissolved solids before DI | Calculate rejection from feed and post-RO TDS; replace when performance falls | £48.99–£56.99 TMC Pure 100 examples |
| DI resin / pod | Polishes remaining ions to the final target | Replace when post-DI TDS rises above 0 or the resin is exhausted | £19.20 TMC 500 ml pod example |
How to tell whether the membrane is still working
Take a feed-water TDS reading and a post-RO reading before DI. Rejection is calculated as ((feed TDS − post-RO TDS) ÷ feed TDS) × 100. For example, 280 ppm feed and 11 ppm after RO is about 96.1% rejection. The exact acceptable figure depends on the membrane and conditions; the point is to watch your own trend, not compare one reading with a forum number.
What does a realistic annual budget look like?
As an illustration, one sediment cartridge (£3.99), one carbon block (£6.99) and two £19.20 DI pods would total about £49.38 before the membrane and water charges. Add a £56.99 membrane every 2–3 years and the simple annualised parts budget is roughly £68–£77. That is a planning example, not a replacement promise: high feed TDS, chloramine, silica or heavy use can exhaust resin much faster.
Do not ignore the waste line
A 3–4:1 waste-to-product ratio is common on conventional systems. Making 100 litres of product water may therefore send roughly 300–400 litres to drain. Collecting that reject water for suitable non-aquarium uses can reduce waste, but never return it to the reef or use it where a contaminant load would be unsafe.
Sources and limitations
Examples were checked against current UK listings for D-D sediment cartridges, D-D carbon cartridges, TMC membranes, TMC replacement parts and Osmotics filter kits. We have not tested cartridge life in a specific UK postcode.