Understanding TDS

TDS stands for Total Dissolved Solids, which includes all dissolved organic and inorganic substances in water. However, the TDS reading does not tell you what is in the water. What is important is what makes up the TDS reading.

For example

100pm of Calcium in water (high TDS) is relatively harmless while 5ppm of copper (low TDS) will kill most aquatic life.

What makes up the TDS value is infinitely more important than the value itself.

Interpreting TDS values

Very high levels of TDS can cause salt stress in plants/livestock. However, unless you know what the TDS level is made up of (what elements are dissolved in the water to produce the TDS level), it is difficult to say at what PPM it becomes harmful. Freshwater, by definition, has less than 1500ppm TDS, while seawater generally has more than 5000ppm. This does not mean that if you have water below 1500ppm it is fine! The advantage of a lower TDS reading is that it generally means you have fewer unknowns in the water column. Tanks with low TDS (below 100ppm) tend to be soft water.

However, if you test your tap water regularly and find that it's TDS value fluctuates over a wide range. (more than 50ppm) this is a good indicator that the tap water is unstable and it would be wise to also test the other values to make sure they are not too far out of range (KH, pH etc).

Adding fertiliser to a planted tank will naturally increase the TDS. In their simple elemental forms most fertilisers are not toxic to livestock unless greatly overdosed.

Some sensitive animals do better in certain ranges. Most commercially available livestock will survive within a very wide range - although increased salt stress from very high TDS levels can shorten the lives of livestock. In controlled tests, white cloud mountain minnows, Corydoras catfish and cherry shrimps survived well in tanks with over 1000 TDS. Natural rivers with very pure water can have TDS levels as low as 5 - 20ppm.

TDS is useful as a test for relative changes.

This relative test can also be used when adding new hardscape or other materials to the pool. If you soak a pile of rocks in a bucket of water and the TDS rises over time, this means that some of the rock is dissolving. Totally inert rock will not change the TDS.

Tanks that use limestone, like mine above, can see high TDS levels of 400+ppm without problems for Neocardina shrimp or plants. Other more sensitive species may prefer a more specific (usually lower) range.

This relative test can also be applied to organic waste. Organic waste will also increase TDS; contrary to what most aquarists think, organic waste doesn't just magically turn into ammonia. The decomposition of organic waste goes through intermediate stages that release carbohydrates, lipids, proteins and nucleic acids into the water column.

A good bacterial cycle will quickly break these down into simpler elements. High levels of undigested organic waste cause algal blooms. Organic waste comes from animals and decaying plants (either plants that are not growing well or old growth), an easy way to get rid of this is to do regular water changes.

If you have a mature tank and are wondering if organic waste is building up in the water column - whether water changes are necessary; measure the rate of increase in TDS over time. Even if ammonia, nitrite/nitrate readings remain at 0, tanks that build up dissolved organic waste over time will show rising TDS levels.

Targeting a minimum value?

Many natural planted biotopes have extremely low TDS ranges; 25ppm and below. Plants are very good at scavenging low levels of nutrients. Plants can grow well by taking up nutrients from the substrate zone even when the water column is very low. The important thing is to ensure that all the nutrients that plants need are present, rather than targeting a particular TDS range in a particular tank.

Brian Jones's tank here runs well on a TDS of just 30ppm.

Examples of TDS values for tanks at the 2hr Gallery:

The tank above has a TDS of around 190ppm. Tap water comes out at 70ppm, and we raise GH to 5dGH using APT Sky. After fertiliser is added, the total TDS is around 180 to 200ppm.

The tank above has a TDS level of around 100. Tap water comes out at 70ppm, and after adding fertiliser it raises the TDS to between 90-100ppm.