What Is TDS In An Aquarium?

TDS (Total Dissolved Solids) is the total amount of dissolved substances in the water. It includes salts, minerals, and other organic materials. However, a TDS number can only tell you how much is dissolved. It does not tell you what those substances are. What matters the most is which substances make up the TDS. 

So, the type of dissolved substances in the water is much more important than the TDS numbers themselves.

Knowing TDS Values

If there are very high TDS (Total Dissolved Solids) levels, these can cause salt stress in plants and aquatic animals. However, the TDS number alone cannot tell you if the water is safe because it does not show what substances are dissolved in the water. For example, freshwater typically has less than 1,500 ppm TDS, while seawater typically has more than 5,000 ppm TDS. However, it does not mean that any water below 1,500 ppm is automatically safe. Water with a low TDS is better because it contains fewer unknown dissolved substances. Water with a TDS below 100 ppm is considered soft water. 

If you regularly test your tap water and notice a TDS change of more than 50 ppm, it indicates that your water supply is unstable. In that case, you should also check other water parameters, such as KH (carbonate hardness) and pH, to ensure they are within safe ranges. 

Adding fertilizers to a planted aquarium can naturally increase the TDS because they add dissolved nutrients to the water. In normal amounts, these fertilizers are generally not harmful to fish or other aquatic animals. 

Here are the different Aquarium Fertilizer Brands Compared (India).

The problems mostly occur only if they are heavily overdosed. Some fish and invertebrates are more sensitive to TDS than others and may prefer specific TDS ranges. However, most commonly kept aquarium species can live in a wide range of TDS values. If TDS are extremely high, they can increase salt stress, which reduces their lifespan over time. 

In controlled tests, most species, including White Cloud Mountain Minnows, Cherry Shrimp, and Corydoras catfish, have been shown to survive well in water with TDS above 1,000 ppm. 

Some natural rivers contain very pure water with TDS levels as low as 5-20 ppm. A TDS meter can tell you how much is dissolved in the water, but not what is dissolved. The actual substances can make up the TDS more important than the TDS number by itself.

How To Use TDS To Check Changes In Your Aquarium?

A TDS meter is most useful for tracking changes over time, instead of deciding if water is "good" or "bad." For example, when you soak new rocks or other decorations in a bucket of water, and TDS generally increases, it means the material is releasing dissolved substances into the water. If the TDS stays the same, the material is inert. It means the material is not dissolving or affecting the water. 

Learn Why Regular Aquarium Water Testing Is Crucial for Your Fish’s Health.

Some aquariums, such as those using limestone, naturally have high TDS levels (around 400 ppm or more). It is usually not a problem for Neocaridina shrimp or many aquarium plants. However, other, more sensitive species can prefer lower TDS levels. TDS can help you check the buildup of organic waste. Most people think organic waste can turn directly into ammonia, but that's not what actually happens. 

As fish waste, uneaten food, and dead plant materials decompose, they first release most of the dissolved organic compounds in the water. These compounds increase the TDS. In a healthy, well-established aquarium, beneficial bacteria can break these compounds into simpler substances. However, if organic waste builds up faster than it can be broken down, it can contribute to algae growth and poorer water quality. The main sources of organic waste are fish and shrimp water, dead plant leaves, and uneaten food. The most effective way to remove dissolved water is through regular water changes. 

A useful way to use a TDS meter is to check how quickly TDS increases over time. If your aquarium's TDS keeps rising between water changes, it shows that the dissolved waste is accumulating, even if nitrite, ammonia, and nitrate all test at zero.

Is There a Minimum TDS You Should Aim For?

There is no minimum TDS value that every aquarium should target. Most natural habitats with a lot of aquatic plants have very low TDS levels. These can sometimes be 25 ppm or even less. Even under these conditions, plants can grow very well because they are efficient at absorbing nutrients. Aquatic plants don't rely on nutrients dissolved in the water. They can also take nutrients from the substrate (the material at the bottom of the tank). It means a tank can have a very low TDS that still supports healthy plant growth.

Instead of trying to reach a specific TDS number, it is important to ensure that all the essential nutrients your plants need are available, whether from the water, the substrate, or fertilizers. So, don't go after a specific TDS number. A low TDS does not always mean plants will struggle, and a higher TDS does not always mean the water is better. You just need to focus on providing a balanced nutrient supply and maintaining stable water conditions.