Where to measure
| Test point | What it tells you | What to do with it |
|---|---|---|
| Tap water | How much dissolved material you’re starting with | Note it monthly. If it rises, your DI resin will run out faster |
| After RO, before DI | How well the membrane is removing dissolved solids | Work out the rejection. If it gets worse, check pressure, temperature and the membrane’s age |
| After DI | Whether the final polish is still working | Treat anything above 0 ppm as your cue to replace the resin or find the fault |
Working out membrane rejection
((tap TDS − post-RO TDS) ÷ tap TDS) × 100 gives you the membrane’s rejection percentage. With 300 ppm from the tap and 9 ppm after the membrane, rejection is 97%. Compare the trend with your membrane’s spec and your own past readings. Don’t judge the membrane by the DI reading, because the resin can hide a failing membrane for a while.
What 0 TDS does and doesn’t mean
DI resin removes the charged particles left after the membrane, which is why a final 0 ppm reading is worth having before you mix salt. But TDS is one overall number based on conductivity. Red Sea’s FAQ points out that nitrate and phosphate can still be detectable when TDS is very low. A meter is no substitute for good maintenance, checking your source water, or testing your tank when something seems wrong.
Common patterns and what they mean
- High tap TDS, normal after RO, 0 after DI: the unit is probably working fine; expect to go through DI resin faster.
- Sudden jump after RO: check pressure, temperature, your flushing routine, how the membrane is seated and the pre-filters.
- Anything above 0 after DI: the resin may be spent, water may be channelling through it, or it may be fitted wrongly.
- Readings jump around: run some water to waste first, rinse the probe, and check the meter is calibrated and measuring flowing water the same way each time.
Keep a record that actually helps
Note the date, tap TDS, post-RO TDS, post-DI TDS, water temperature, pressure if you can measure it, and when each cartridge was changed. Then “the water seems off” becomes a problem you can trace. You can keep these alongside your tank readings in the free Reef Companion.
Sources and limits
Testing at three points matches how SpectraPure describes monitoring tap, RO and DI water. The point that low TDS isn’t a complete contaminant test comes from Red Sea’s salt FAQ. This is a practical guide, not a replacement for a lab analysis of your water.
THE SHORT ANSWER
TDS tells you how your filters are doing, not what’s in the water
A TDS meter estimates how much dissolved, charged material is in water by measuring its conductivity. It can’t tell you which substances are there, pick up every contaminant, or prove water is free of microbes. Its best use for a reefer is comparing readings from your tap, after the membrane and after the DI, to see whether each stage is still doing its job.
Your tap reading gives you a starting point, the post-RO reading lets you work out membrane rejection, and the final reading shows whether the DI resin is still hitting your target. Read a zero within the limits of your meter’s resolution and calibration, not as proof that nothing at all is in the water.
STEP BY STEP
How to do it
- 01
Test at the right points
Measure tap water, water after the membrane and the final water, using clean containers. Keep salt residue, wet hands and shared utensils away from the sample.
- 02
Let the reading settle
Run some water to waste as your unit’s instructions suggest, and give the meter time to settle. Temperature compensation and calibration affect whether readings can be compared.
- 03
Work out the rejection
Compare tap and post-RO TDS in similar conditions. The trend tells you more than any one reading.
- 04
Match the result to a stage
High final TDS with good rejection points to the DI resin. Poor rejection after the membrane means checking pressure, temperature, the flow restrictor and the membrane.
YOUR SITUATION
What to do in your situation
It shifts with the seasons or your water supply
Update your baseline and keep an eye on how fast filters and resin run out.
Rejection gets worse across similar tests
Check the running conditions and the membrane.
The DI no longer gets you to your target
Replace or fix the DI stage before you make any reef water.
COMMON MISTAKES
Mistakes to avoid
Testing in a salty cup
A trace of salt can give a big TDS reading and send you after the wrong problem.
Treating low TDS as safe
A low number won’t pick up metals, disinfectants, microbes or uncharged substances.
Skipping the post-RO reading
A final zero can hide a membrane that’s weakening, while your resin costs quietly rise.
WORKED EXAMPLE
Reading three numbers together
Tap water is 320 ppm, water after the membrane is 10 ppm and the final water is 0 ppm. Membrane rejection is about 96.9%, and the DI resin is finishing the job. Note the pressure and temperature too, and you have a useful baseline.
Two months later, the post-RO reading is 24 ppm and the final water still reads 0. The end result looks fine, but the resin is now dealing with more than twice the load. Sort out the membrane before the DI stage starts letting things through.
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 TDS should RODI water be?
Most reefers aim for a steady zero on their meter after the DI, while accepting the meter’s limits.
Can a TDS meter detect chlorine?
No, it can’t pick out chlorine or chloramine specifically. Follow the product’s and your water company’s guidance on carbon filtration.
What is TDS creep?
Water that’s been sitting at the membrane can read higher when you first turn the unit on. Follow your unit’s flushing guidance and judge the water once it’s running steadily.