Replace the sediment and carbon filters every 6 to 12 months (every 6 if your city uses chloramine or you make a lot of water), the RO membrane every 2 to 3 years or when its rejection rate drops, and the DI resin as soon as a TDS meter reads anything above 0 after the DI stage. That’s the guidance from Bulk Reef Supply, a large reef equipment retailer, and it matches what most reef keepers do. A TDS meter turns the membrane and DI decisions from guesses into readings.
RO/DI filter replacement schedule #
| Stage | What it removes | Replace when | Typical interval |
|---|---|---|---|
| Sediment filter | Sand, rust, particles | Water pressure or flow drops | 6-12 months |
| Carbon block | Chlorine, chloramine, organics | On schedule, or when chlorine shows in the waste water | 6-12 months |
| RO membrane | Most dissolved solids | Its rejection rate falls noticeably | 2-3 years or more |
| DI resin | The dissolved solids the membrane missed | Any TDS above 0 after the DI stage | Depends on source water and membrane health |
Use the short end of each range if your tap water is hard, dirty or chloraminated, or if you run the unit hard for a large system.
How do you tell when each RO/DI filter is done? #
Sediment filter: watch the pressure #
A clogged sediment filter chokes pressure and flow to everything after it, which makes the membrane work worse and slows production. If your unit has a pressure gauge, note the reading with fresh filters and replace the sediment stage when it drops noticeably. If production slows for no obvious reason, check the sediment filter first.
Carbon block: go by the calendar #
Carbon is the hardest stage to check, because there’s nothing to see or easily measure. It’s also the stage that protects the membrane from chlorine, which damages membranes. BRS recommends replacing it on a schedule, every 6 to 12 months, and suggests chlorine test strips on the waste water line as a check. If your utility uses chloramine, which standard carbon removes less easily, stay at the 6-month end or use a carbon made for chloramine.
RO membrane: calculate the rejection rate #
With a dual TDS meter (one probe before the membrane, one after), you can work out how much the membrane removes:
rejection rate = (TDS in − TDS after membrane) ÷ TDS in × 100
For example, tap water at 300 ppm and membrane output at 9 ppm gives (300 − 9) ÷ 300 × 100 = 97%. BRS says a healthy membrane can approach 99% under ideal conditions. When the rate falls and your DI resin starts running out much faster than it used to, the membrane is due. BRS puts a typical membrane’s life at 2 to 3 years or more with good prefilter care.
DI resin: zero is the only acceptable reading #
The DI stage should put out 0 TDS. Anything above that means the resin is exhausted, and BRS says anything other than 0 isn’t considered safe for a reef. If you don’t have a TDS meter, color-changing resin shows exhaustion as a band of color moving up the cartridge from the bottom. Change it before the band reaches the top.
Why does chloramine shorten carbon life? #
Many water utilities disinfect with chloramine instead of chlorine because it lasts longer in the pipes. That same stability makes it harder for ordinary carbon to remove, so more of it reaches the membrane, and part of it breaks down into ammonia that the membrane and DI resin then have to handle. Your utility’s annual water quality report says which disinfectant it uses. If it’s chloramine, plan for more frequent carbon changes or an extra carbon stage.
How do you change RO/DI filters step by step? #
- Shut off the water supply to the unit and open the product line to release pressure.
- Unscrew each housing with the housing wrench. Keep a towel handy.
- Swap the cartridges, checking the flow direction marked on each one.
- Check the O-rings. Wipe them clean and replace any that are flattened or cracked. A thin coat of food-grade silicone grease helps them seal.
- Flush the new sediment and carbon filters for about 60 seconds through the flush valve, as BRS suggests, so carbon fines don’t reach the membrane.
- Let the new DI resin run for a gallon or two before you collect water for the tank, then confirm 0 TDS.
- Write down the date and the TDS readings.
How do you keep track of RO/DI filter changes? #
The trouble with six- and twelve-month jobs is that nobody remembers when they last did them. Write the install date on each cartridge with a marker, and keep a record somewhere you’ll see it.
Fish Tank Tracker has an equipment log where you can add your RO/DI unit with its install date, then attach recurring maintenance tasks with custom intervals: sediment and carbon every 180 days, a membrane check once a year, a TDS check every month. The app sends a local notification when each task comes due, and you can keep your TDS readings in the task notes. The equipment log and reminders are free, and your data never leaves your phone.
Good RO/DI water matters most in two places: mixing salt, covered in how much salt per gallon, and top-off water, which has to be pure for the reasons in what salinity a reef tank should be. For the rest of the upkeep calendar, see our fish tank maintenance schedule.
Frequently asked questions #
Can I use tap water in a reef tank instead of RO/DI? #
Some fish-only keepers do, but most reef keepers don’t. Tap water can carry phosphate, nitrate, silicate, copper and chloramine, which feed nuisance algae or harm invertebrates. Test your tap water and read your utility’s report before deciding.
What TDS should RO/DI water be for a reef? #
Zero after the DI stage. Water straight from the membrane usually reads a few ppm, which is why the DI stage is there.
How long does DI resin last? #
It depends on how much water you make, your tap water’s TDS and how well the membrane is working. With a healthy membrane, resin lasts much longer. When a membrane is failing, resin can exhaust in weeks, and that’s often the first sign.
Do I need RO/DI for a freshwater tank? #
Usually not. Dechlorinated tap water works for most freshwater fish. RO water helps with soft-water species like discus, for shrimp keepers who remineralize to a set hardness, and where tap water is high in nitrate or phosphate.