TDS

TDS stands for Total Dissolved Solids. It literally translates to "Total Dissolved Matter." In our case, you're measuring the amount of dissolved substances in the pond water. According to the dictionary, "dissolving" means "forming a homogeneous mixture with a liquid." If we take that as truth, the term "total" isn't entirely accurate. The TDS meter only measures dissolved substances that ionize. This means (for convenience) that these substances dissolve into molecules that acquire a charge (positive or negative). For example, salt dissolves in water and forms two molecules: a sodium ion (Na+) and a chlorine ion (Cl-). Salt dissolved in water is therefore detected by the TDS meter. Sugar also dissolves in water, but these remain sugar molecules and are therefore not detected by the TDS meter.

How does a TDS meter work?

A TDS meter measures the conductivity of pond water. It contains two electrodes, to which a small voltage is applied. The more substances are dissolved in the water, the easier it is to transfer the voltage to the second electrode. Contrary to popular belief, pure water (which is different from drinking water!) conducts very little current. It is the substances dissolved in the water that conduct the current from electrode 1 to electrode 2. The conductivity is expressed in milligrams per liter or parts per million (ppm). The higher the value, the more dissolved substances are present in the pond water.

What's in it for me?

A high TDS value can indicate a polluted pond. A polluted pond can lead to fish disease. Ammonium, nitrite, nitrate, and phosphate, for example, cause TDS levels to rise. You don't want to let them rise (too high).

So I'm aiming for the lowest possible TDS value?

No, not quite. There are substances dissolved in our tap water that are indeed important for the health of the pond. (Bi)carbonates, for example, form the essential parameter KH (carbonate hardness). The trick is to compare the pond's TDS value with that of tap water. Tap water only contains "good" substances dissolved in it. Anything added to the pond is considered "bad" substances (think ammonium, nitrite, nitrate, and phosphate). The closer the pond's TDS value is to that of tap water, the better!

Which standard should I use?

Since tap water in the Netherlands comes from various suppliers, it can't be expressed in a single figure. The KH and GH values ​​are higher at one water supplier than at another. Therefore, the standard is "K + (maximum) 200." The K represents the TDS value of the tap water you use to refresh the pond. Above that, your TDS value may exceed 200 ppm. A maximum of 100 ppm is even better.

If the TDS of your tap water is 200, the TDS of your pond shouldn't exceed 400 (preferably 300). If it does, you'll need to change more water.

What is the added value of my complete test kit?

If you measure ammonia, nitrite, nitrate, and similar substances separately, a TDS meter is unnecessary. After all, you'll know the level of contamination. However, it can be a useful, quick measurement that gives you an idea of ​​the level of contamination. A TDS meter isn't essential, but rather an additional tool.

I have salt in the pond…

While I personally don't see the point, some people have permanently dissolved salt in their ponds. In that case, stop taking TDS measurements, because salt drives the TDS level to enormous heights, making it impossible to accurately assess the level of pollution.

Author: Joop van Tol (www.koitoday.nl)