Understanding TDS in the Aquarium

Introduction

TDS stands for Total Dissolved Solids, a measure of the combined concentration of all dissolved substances in your water: minerals, salts, metals, organic compounds, and anything else dissolved in the water column. It is measured in parts per million (ppm) or milligrams per liter (mg/L), and a TDS meter is one of the cheapest and most informative test tools available to hobbyists.

For most community fishkeepers, TDS is a secondary concern compared to ammonia, nitrite, nitrate, and pH. But for shrimp keepers, discus keepers, and anyone working with sensitive or wild-caught fish, TDS is an essential parameter that the standard test kit suite does not measure.

Quick Overview

What TDS Measures (and What It Does Not)

TDS measures the total quantity of dissolved ions in the water but does not identify what those ions are. Two tanks with the same TDS reading could have very different water chemistry: one might be rich in calcium and magnesium (hard water), while the other has high sodium chloride or elevated organic compounds. TDS gives you a total number, not a breakdown.

This is why TDS is used alongside, not instead of, specific tests for GH, KH, and pH. In practice, TDS correlates reasonably well with general hardness in most tap water, which is why shrimp keepers use it as a quick daily check.

A TDS meter is a $10-15 investment and gives you an instant reading in seconds. For shrimp keepers who need to check water parameters before and after water changes, it is faster and more convenient than a liquid test kit for routine monitoring.

TDS Ranges for Common Setups

• RO/DI water (remineralized for Caridina shrimp): 100-150 ppm. Precisely controlled mineral content for demanding shrimp.
• Neocaridina shrimp tanks: 150-250 ppm. Wider tolerance, typical moderately hard tap water range.
• Typical community fish tank: 150-350 ppm. Most fish do well across this range provided GH, KH, and pH are appropriate.
• Blackwater biotopes: 50-120 ppm. Very low mineral content, characteristic of soft blackwater environments.
• African cichlid tanks: 300-600+ ppm. High mineral content characteristic of Rift Valley lake water.

Why Shrimp Keepers Monitor TDS Closely

Freshwater shrimp are highly sensitive to osmotic pressure, the difference in dissolved solid concentration between the water and their body fluids. When TDS shifts rapidly, the shrimp's cells must work to re-equalize, and if the shift is too large too fast, it triggers a molting event or causes osmotic shock.

This is why drip acclimation is so important for shrimp, and why water changes in shrimp tanks should use water prepared to match the tank TDS closely. A 10-15% water change with water that is 50 ppm different from the tank is usually fine. A 30% change with water 150 ppm different from the tank is a recipe for mass molting or deaths.

• Test TDS of new water before adding it to a shrimp tank
• If TDS has crept up (from evaporation concentrating dissolved solids), top off with RO or distilled water to bring it back down rather than doing a large water change
• A sudden unexplained TDS spike often means something is dissolving in the tank: new rock, decaying organic matter, or a new fertilizer

Using a TDS Meter Correctly

TDS meters are simple to use but a few best practices improve accuracy.

• Calibrate your meter with a calibration solution when you first receive it and every few months thereafter
• Rinse the probe with distilled water before and after each use to prevent cross-contamination
• Take readings at consistent temperature, as TDS meters compensate for temperature but some calibration drift occurs with large temperature differences
• Measure the TDS of your tap water and track it over time; municipal water TDS can vary seasonally as water sources change