What does TDS mean in an aquarium?

Why this happens

Every dissolved ion contributes to conductivity. Calcium, magnesium, sodium, nitrate, bicarbonate and treatment salts can all change the reading, so TDS cannot identify which substance increased.

• Evaporation concentrates dissolved material.
• Mineral-rich top-ups raise the reading without removing waste.
• Remineralising salts deliberately increase conductivity.
• Nitrate and other accumulated ions can contribute to drift.

TDS is especially useful for repeatable RO remineralisation and shrimp systems when paired with GH, KH and source control. It is less useful when interpreted as a universal purity score.

What to check first

• Is the meter calibrated and temperature compensated?
• What is the source-water reading before treatment?
• Do GH, KH and nitrate explain the trend?
• Was the sample taken at the same temperature and time?

Compare the result with the aquarium’s normal baseline, not only a generic target. Record the time, recent maintenance, new livestock, feeding changes, source-water details and any equipment fault. A short timeline often separates the original cause from secondary symptoms.

A practical action plan

1. 1. Calibrate the meterUse the specified calibration solution and clean the probe.
2. 2. Establish a baselineRecord source water, prepared water and aquarium water under normal conditions.
3. 3. Investigate changesUse targeted tests to identify whether minerals, nitrate or an additive changed.
4. 4. Correct the causePerform appropriate water changes or adjust the preparation recipe gradually.

How to interpret the response

A stable reading supports recipe consistency but does not prove water is safe. A rising trend tells you dissolved material is accumulating; other tests identify what matters biologically.

Change one important variable at a time where welfare allows, then observe and retest. When several products or adjustments are made together, it becomes difficult to know what helped, what caused stress and whether the underlying problem remains.

What to avoid

Do not equate TDS with hardness, use an uncalibrated pen to make large changes, compare readings from different units without conversion or lower TDS rapidly in sensitive livestock.

When it is urgent

A sudden large reading change after dosing, water change or top-off—especially with shrimp distress or loss of balance—requires immediate verification and source investigation.

How to prevent it next time

Rinse and calibrate the meter, record units, replace evaporation with suitable water and keep a written remineralisation recipe.

Keep enough records to recognise the aquarium’s normal behaviour, water chemistry and equipment performance. Trends are more useful than isolated numbers, and early correction is usually safer than a large emergency change.