TDS (total dissolved solids)

TDS measures the total quantity of substances dissolved in water. It is mainly used to check that the reverse osmosis system is working correctly.

Detailed explanation

The TDS metre is an economical instrument (15-20 EUR) that measures the electrical conductivity of water and converts it into ppm of dissolved solids. It doesn't tell you what's in the water, only how much there is.

For RO water TDS is the key indicator. If it leaves the system at 0-5 ppm the membrane works. If it comes out at 30 ppm the membrane is failing.

For freshwater aquariums with caridina the TDS of the water in the aquarium is a management parameter. The crystal reds want TDS 100-150 ppm. Neocaridins tolerate 200-300 ppm. Too high a TDS indicates mineral buildup.

For marine aquariums, the TDS of the water in the aquarium cannot be measured (it would be very high due to the salts). Only the incoming RO water and top-up water are measured.

In ponds the TDS of mains water gives an idea of ​​the quality. Water below 200 ppm is generally good. Above 500 ppm there are many minerals and could use treatment.

Reference values by aquarium type

How it is measured

TDS immersion metre (a few seconds). It costs 10-20 EUR. Calibrate with reference solution once a month.

What happens when it is too low

For freshwater aquariums: water too pure, missing essential minerals for fish. Remineralize.

What happens when it is too high

For RO water: membrane to be replaced. For Caridina aquariums: risk of failed moult. For general freshwater aquariums: mineral accumulation, do more water changes.

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