Guide
TDS and aquarium water: what total dissolved solids actually mean
Portable TDS pens read electrical conductivity through your water sample and estimate “ppm TDS.” That number is invaluable for repeatable remineral mixes - and dangerously easy to misunderstand as a stand-in for fish-safe chemistry tests.
- TDS is a conductivity proxy, not a chemistry report: it tells you how much stuff is dissolved, not what that stuff is - calcium, nitrate, tannins, and fertiliser all push the number up.
- It does not replace your test kit: ammonia, nitrite, nitrate, chlorine, and copper are electrically invisible or unreliable on a TDS pen - test them properly.
- GH and TDS are related but not the same: tannins, sulphate, sodium, and potassium raise TDS without meaningfully moving a GH drop test.
- Evaporation concentrates ions: TDS will drift upward between water changes as water evaporates but dissolved solids stay behind - top off with pure water, not tank water.
- Best use cases: confirming RO membrane performance, keeping remineralisation blends consistent batch to batch, and spotting unexplained parameter drift early.
- Shrimp TDS targets are real but often misunderstood: species-specific ranges exist, but the number alone tells you nothing about mineral balance - a mix of the wrong salts at the right TDS will still kill a colony.
What a TDS meter actually measures
A TDS pen does one thing: it passes a tiny alternating current between two electrodes and measures how readily the water conducts it. More dissolved ions means more conductivity. The meter's firmware then converts that raw conductivity figure - expressed in microsiemens per centimetre (µS/cm) - into a parts-per-million (ppm) number using a fixed conversion factor, typically assuming a sodium chloride equivalent.
That last part matters more than most guides admit. Two different pens dipped into the same water can give readings that differ by 10-20 ppm, not because one is broken but because they use slightly different conversion factors. This is why you should pick one meter, stick with it, and log relative changes rather than treating the absolute number as gospel. What matters for your tank is drift from your established baseline, not whether your reading matches a stranger's on a forum.
Temperature also affects conductivity - warmer water reads higher. Better pens apply automatic temperature compensation (ATC), but budget models do it imperfectly. If you test cold tap water and warm tank water back to back, expect a few ppm of phantom difference. Test at consistent temperatures when precision matters.
Calibrate your pen with the calibration solution specified by the manufacturer - usually 342 ppm or 1413 µS/cm NaCl standard - and re-check it every few months. A pen that reads 400 ppm in 342 ppm standard is lying to you consistently, which is worse than being slightly off.
TDS versus GH: why they diverge
This is where most beginners get confused, and honestly where a lot of intermediate fishkeepers go wrong too. GH (general hardness) measures primarily calcium and magnesium ions - the minerals responsible for hardness in the biological sense that matters for fish osmoregulation, breeding triggers, and invertebrate shell formation.
TDS measures everything that conducts electricity: calcium, magnesium, yes - but also sodium, potassium, chloride, sulphate, nitrate, phosphate, and the ionic breakdown products of tannins from driftwood and botanicals. You can have a tank with low GH and high TDS if it's full of botanicals and dosed with potassium fertiliser. You can also have high GH and relatively modest TDS if the water is simply hard with calcium carbonate and not much else.
The practical upshot: TDS cannot tell you whether your water is appropriately hard for your livestock. A breeders' pair of Pterophyllum scalare (angelfish) wants soft water around 3-8°dGH - TDS won't confirm that. A colony of Caridina shrimp needs specific mineral ratios, not just a particular number. Always run GH and KH tests alongside TDS when mineral balance actually matters for your species.
Most community tanks in the 150-400 ppm TDS range kept with locally-conditioned tap water never need a TDS meter at all. It becomes genuinely useful when you start working with RO water, soft-water species, or shrimp breeding - not as an everyday tool for a planted guppy tank.
When a TDS meter actually earns its place
Verifying RO membrane performance
A functioning reverse osmosis membrane should strip your source water down to somewhere between 0 and 20 ppm. If you fill your RO reservoir and read 80 ppm, your membrane is degrading and needs replacing. This is probably the single most objectively useful thing a TDS pen does - a 30-second test that tells you whether a £150+ membrane is still doing its job. Test your RO output monthly.
A reading above 20-30 ppm from a dedicated RO unit suggests the membrane is spent or the system has a bypass leak.
Keeping remineralisation blends consistent
When you mix remineralisation salts - products like Seachem Equilibrium, Salty Shrimp GH/KH+, or similar - into RO or distilled water, TDS gives you a fast, repeatable check that each batch matches the last. You dial in a target on your first successful batch (say, 180 ppm for a soft-water planted tank), then hit that number on every refill without weighing salts on a scale every time. It's a consistency tool, not a chemistry analyser.
Spotting mystery drift
If your tank parameters feel off and you can't pin down why, a TDS baseline can help you rule things in or out. An unexplained rise of 50+ ppm from your usual baseline tells you something is dissolving into your water that wasn't there before - a new piece of hardscape leaching calcium, a fertiliser overdose, substrate breakdown. It won't tell you what, but it narrows the field before you run through your whole test kit.
Evaporation is the most common cause of steady TDS creep. Water evaporates; dissolved ions don't go anywhere. Between water changes in a warm, open tank, TDS can climb noticeably - which is why you top off with pure RO or dechlorinated tap water (not mixed tank water) to maintain concentration, then do your regular water changes to actually remove accumulated solids.
Never use TDS as a substitute for ammonia, nitrite, or nitrate testing. These compounds have minimal electrical conductivity relative to the other ions in your tank - a TDS pen cannot detect a dangerous ammonia spike. If your tank isn't fully cycled, run proper liquid tests. See our guide on how to cycle a fish tank if you're at that stage.
The shrimp TDS rabbit hole
Neocaridina and Caridina shrimp keeping has built an entire subculture around TDS targets, and some of it is genuinely useful - some of it is folklore that's been repeated until it sounds like science.
The useful part: Neocaridina species (cherry shrimp and variants) generally thrive in 150-250 ppm TDS, while Caridina (Crystal Red, Taiwan Bee, etc.) typically want 100-180 ppm using RO water remineralised with a shrimp-specific mineral blend. These ranges correlate with the water chemistry their wild habitats provide.
The folklore part: hitting the TDS number does not mean you have the right water. 150 ppm of sodium chloride is not the same as 150 ppm of balanced calcium, magnesium, and potassium. Shrimp breeders who succeed long-term are controlling the mineral source and ratio, then using TDS as a consistency check - not chasing a number on a pen. If you're using a dedicated shrimp remineraliser at the manufacturer's recommended dose and verifying with TDS, you're doing it right. If you're adding random aquarium salts until the pen reads what a forum post told you, you're not.
Adding tap water conditioner, fertilisers, or Indian almond leaves and then comparing your TDS reading to RO-based shrimp tank targets from a forum. These numbers are not comparable - the sources of those dissolved solids are completely different, and target ranges cited for Caridina shrimp assume RO-remineralised water, not conditioned tap water.
What TDS cannot tell you - and what to test instead
A recurring problem is fishkeepers buying a TDS pen and treating it as a general water quality meter. It isn't. Here's what it genuinely cannot assess:
- Ammonia and nitrite - the toxic products of an uncycled or overloaded tank. Understand the nitrogen cycle and test these with liquid reagent kits.
- Nitrate - electrically present but drowned out by other ions at typical tank concentrations; a TDS pen is not a reliable nitrate proxy.
- Chlorine and chloramine - your dechlorinator handles this, but a TDS pen cannot confirm it worked.
- Dissolved oxygen - zero conductivity contribution; completely invisible to a TDS meter.
- pH - pure water at neutral pH conducts almost nothing; acidic or alkaline solutions conduct differently, but pH is not what TDS reads. Use a proper pH meter or test kit.
- Copper - lethal to invertebrates and shrimp at trace concentrations; must be tested with a copper-specific kit, especially in tanks on older plumbing.
For a full picture of the parameters that actually govern fish health, read our overview of aquarium water parameters.
Practical calibration and maintenance
TDS pens are low-maintenance but not zero-maintenance. Electrode fouling - usually mineral scale or organic film - causes readings to drift low over time. Rinse the electrode cap thoroughly after each use and store dry. If readings seem implausibly low on a known sample, clean the electrodes gently with a cotton bud dampened with white vinegar, rinse with distilled water, and re-calibrate.
Most pens use a small button cell battery. Weak batteries affect reading accuracy before the pen displays a low-battery warning. If your readings start behaving erratically, try a new battery before you blame the water.
For logging purposes, App-aquatic lets you record TDS alongside GH, KH, pH, and temperature in a single test entry - useful for building the baseline history that makes drift meaningful rather than just alarming.
Test and log your source water (tap or RO) TDS every time you test your tank. If your tap TDS shifts - and municipal supplies do fluctuate seasonally - you'll know immediately whether a tank reading change is coming from your source or from something happening inside the tank.
Track your parameters and get care reminders with App-aquatic.
Get the free appWhat TDS level is safe for fish?
There's no single answer - it depends on the species. Most hardy community fish (tetras, livebearers, corydoras) tolerate 100-400 ppm without issue when the underlying mineral balance is appropriate. What matters more than the TDS number is whether the water's hardness and pH match your fish's requirements. A high TDS from nitrate and organics in a neglected tank is a problem regardless of the number; the same TDS from well-mineralised RO water is fine.
Can I use a TDS meter instead of a water test kit?
No. A TDS meter cannot detect ammonia, nitrite, chlorine, dissolved oxygen, or pH - the parameters most likely to kill your fish if they go wrong. It's a supplementary tool, not a replacement. At minimum, run liquid tests for ammonia, nitrite, nitrate, and pH, especially on any tank that isn't fully cycled.
My TDS is rising steadily between water changes - is that a problem?
It's normal and expected. Evaporation removes water but leaves dissolved solids behind, so TDS creeps up. Top off evaporation losses with pure water (RO, distilled, or dechlorinated tap water at your baseline TDS), and do regular water changes to export accumulated solids. If TDS is rising faster than evaporation explains, check for new hardscape, substrate changes, or a fertiliser overdose.
Why do two TDS pens give me different readings on the same water?
Different pens use different NaCl-equivalent conversion factors to translate raw conductivity into ppm. A reading of 210 ppm on one pen might be 230 ppm on another - both are accurately measuring conductivity, they're just applying different maths. Pick one pen, calibrate it properly, and log your tank's readings relative to that pen's baseline. Cross-brand comparisons are not meaningful.
Do I need a TDS meter for a basic community tank on tap water?
Honestly, probably not. If you're keeping hardy community fish from tap water - something like the species on our best beginner fish list - your energy is better spent on a good liquid test kit and consistent water changes. TDS meters pay off when you're working with RO water, soft-water breeding projects, or shrimp. For a straightforward community setup, it's a nice-to-have, not a necessity.
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