TDS in Aquariums: What It Means and When It Matters

Table of Contents

• What Does TDS Actually Measure?
• TDS Meters: How They Work
• Singapore Tap Water TDS
• When TDS Matters: Shrimp Keeping
• TDS Creep from Evaporation
• Remineralising RO Water
• TDS, GH and KH Relationship
• When NOT to Worry About TDS
• Frequently Asked Questions

What Does TDS Actually Measure?

TDS stands for total dissolved solids. It is a measurement of everything dissolved in your water — every mineral, salt, metal, organic compound, fertiliser residue, fish waste byproduct and chemical additive combined into a single number expressed in parts per million (ppm).

This is both the strength and the weakness of TDS as a measurement. It tells you how much “stuff” is in your water, but it tells you nothing about what that stuff is. A TDS reading of 200 ppm could mean your water contains beneficial minerals at ideal concentrations, or it could mean it is loaded with nitrates and fertiliser residue. The number alone does not distinguish between the two.

TDS is therefore best understood as a general indicator — a single number that tracks overall water purity and consistency. It is most useful when you know what is contributing to the reading and can interpret changes in context.

TDS Meters: How They Work

A TDS meter (also called a TDS pen) is a simple, affordable device that measures the electrical conductivity of water and converts it to an estimated TDS value in ppm. Dissolved ions conduct electricity, so higher conductivity means more dissolved substances.

Using a TDS Meter

1. Turn on the meter and dip the electrode end into your water sample.
2. Wait for the reading to stabilise (5-10 seconds).
3. Read the ppm value on the display.
4. Rinse the electrode in clean water between measurements to avoid cross-contamination.

Choosing a TDS Meter

Basic TDS pens are available from aquarium shops, hardware stores and online retailers in Singapore for $10-25 SGD. Popular brands include HM Digital, Xiaomi and generic models. For aquarium purposes, an inexpensive pen is perfectly adequate — laboratory precision is not necessary for hobbyist use.

Calibrate your meter periodically using calibration solution (usually 342 ppm NaCl solution) to ensure accuracy over time.

Singapore Tap Water TDS

Singapore’s PUB-treated tap water has remarkably low TDS, typically ranging from 30-60 ppm. This is significantly lower than tap water in most other countries and reflects the high quality of Singapore’s water treatment infrastructure.

The low TDS of Singapore tap water means it is relatively “clean” as a starting point and close to RO water in mineral content. For many hobbyists, this is an advantage — you are starting with a nearly blank canvas that you can remineralise to your exact specifications.

When TDS Matters: Shrimp Keeping

TDS is most commonly monitored by shrimp keepers, where consistent water parameters are critical to health and breeding success.

Caridina Shrimp (Crystal Red, Crystal Black, Taiwan Bee)

Caridina shrimp keepers typically use 100% RO water remineralised with a Caridina-specific product like SaltyShrimp GH+ to achieve these precise targets. TDS is used as a quick daily check to confirm that water parameters remain consistent.

Neocaridina Shrimp (Cherry, Blue Dream, etc.)

Neocaridina are hardier and tolerate a wider TDS range, but consistency matters more than hitting a specific number. Avoid sudden swings of more than 20-30 ppm between water changes.

How Shrimp Keepers Use TDS

For shrimp keepers, TDS serves as a convenient shortcut. Once you know that a TDS of, say, 110 ppm corresponds to your target GH and KH (because you are remineralising from pure RO water), you can use the TDS pen for quick daily checks rather than running full GH/KH tests every time. If TDS drifts up, it signals that something is accumulating. If it drops after a water change, you can verify that your replacement water matches the tank parameters.

TDS Creep from Evaporation

Evaporation removes pure water from your tank but leaves all dissolved solids behind. Over time, this concentrates the minerals, fertilisers, and waste products in your water, causing TDS to rise steadily between water changes. This phenomenon is called TDS creep.

In Singapore, evaporation rates can be significant due to ambient heat (28-32 °C) and the use of open-top tanks, fans and surface agitation. A tank that loses 5% of its water volume to evaporation over a week will see a corresponding rise in TDS even if nothing else changes.

Managing TDS Creep

• Top off with pure water only. When replacing evaporated water, use RO or distilled water — not tap water. Tap water adds minerals on top of the already concentrated tank water, accelerating the rise.
• Regular water changes. Water changes are the only way to actually reduce TDS. A 20-30% weekly water change with appropriately remineralised water resets the concentration.
• Monitor with your TDS pen. Track TDS between water changes. If it climbs more than 20-30 ppm above your target, schedule a water change sooner.

Remineralising RO Water

Reverse osmosis (RO) water has near-zero TDS, GH and KH — it is essentially pure H2O. While this purity is useful as a starting point, pure RO water is unsuitable for aquarium use directly. Fish and shrimp need dissolved minerals for biological processes, and the complete absence of buffering capacity means pH can swing dangerously.

How to Remineralise

1. Prepare RO water in a separate container (not directly in the tank).
2. Add a remineraliser product (SaltyShrimp GH+, GH/KH+, or Seachem Equilibrium) according to the manufacturer’s dosing instructions.
3. Stir thoroughly and let it dissolve completely.
4. Test TDS (and ideally GH) to confirm you have reached your target.
5. Use the remineralised water for water changes and top-offs.

Remineraliser Products

Since Singapore tap water already has low TDS (30-60 ppm), some hobbyists use it directly with a small addition of remineraliser rather than investing in an RO unit. This is a practical and cost-effective approach for Neocaridina and community tanks.

TDS, GH and KH Relationship

TDS, GH and KH are related but not interchangeable measurements.

• GH contributes to TDS (calcium and magnesium are dissolved solids), but TDS also includes many things that GH does not measure — sodium, potassium, nitrate, chloride, fertiliser compounds, etc.
• KH also contributes to TDS (carbonate and bicarbonate ions are dissolved solids), but again, TDS includes far more than just KH.
• TDS is the sum of everything. If you add fertiliser to your tank, TDS rises even though GH and KH remain unchanged.

This is why TDS alone is an unreliable indicator of water quality. A TDS of 200 ppm in a remineralised shrimp tank (where you know exactly what contributes to that number) is very different from a TDS of 200 ppm in a heavily fertilised planted tank where half the reading comes from fertiliser residue and accumulated nitrates.

For a deeper exploration of GH and KH specifically, see our GH and KH guide.

When NOT to Worry About TDS

TDS is frequently overemphasised, especially among newer hobbyists. Here are situations where TDS monitoring is not particularly useful.

Community Fish Tanks

Most tropical fish — tetras, rasboras, bettas, gouramis, corydoras — are highly adaptable to a wide range of TDS. They thrive in Singapore tap water (TDS 30-60 ppm) just as well as they do in harder water. Monitoring TDS in a standard community tank adds no actionable information beyond what regular water changes and basic water testing already provide.

Planted Tanks with Fertiliser Dosing

If you are dosing liquid fertiliser, your TDS will rise after each dose and drop after each water change. This is entirely expected and does not indicate a problem. The fertiliser minerals are beneficial for your plants. Chasing a specific TDS target in a fertilised tank is counterproductive.

Fish-Only Tanks

In tanks without shrimp, snails, or particularly sensitive species, TDS monitoring offers little practical value. Focus your testing on ammonia, nitrite, nitrate and pH — these are the parameters that directly affect fish health.

Frequently Asked Questions

What is a “good” TDS for a general aquarium?

There is no universal ideal. For community fish tanks, anywhere from 50-400 ppm is acceptable and most tropical species will be fine. TDS targets only become critical for shrimp keeping, where species-specific ranges apply. Focus on stability and consistency rather than chasing a specific number.

My TDS keeps rising even with regular water changes. Why?

Several factors can cause persistent TDS rise: fertiliser accumulation, evaporation concentrating solids, dissolving minerals from rocks or substrate (especially crushed coral, limestone, or seiryu stone), and fish waste. If TDS rises significantly between water changes, consider larger or more frequent water changes, and check whether your hardscape is contributing dissolved minerals.

Do I need an RO unit in Singapore?

For most hobbyists, no. Singapore tap water has very low TDS (30-60 ppm) and is naturally soft. For Neocaridina shrimp and community fish, tap water with a GH/KH remineraliser is sufficient. An RO unit is most beneficial for Caridina shrimp keepers who need precise, ultra-consistent parameters starting from a zero baseline.

Does activated carbon affect TDS?

Activated carbon can lower TDS slightly by adsorbing certain organic compounds. However, the effect is modest and temporary. Carbon is more useful for removing tannins, odours, and medications than for TDS management.

Monitor What Matters

TDS is a useful tool in the right context — particularly for shrimp keeping, RO water preparation and tracking water consistency between changes. But it is not a magic number, and for most community fish and planted tanks, regular water changes and basic parameter testing (ammonia, nitrite, nitrate, pH) matter far more.

If you are setting up a shrimp tank or want help understanding your water parameters, visit us at Gensou, 5 Everton Park, Singapore. We stock TDS meters, remineralisers and everything you need for precise water management. Our maintenance team can also help you establish a testing and water-change routine tailored to your specific setup.

Related Reading

• Aquarium TDS Explained for Shrimp Keepers: Ideal Ranges
• TDS Guide for Shrimp Keepers: Measuring What Matters
• Active vs Inert Substrate: Which Is Right for Your Planted Tank?
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