What is a TDS meter and why does it matter?


A TDS meter is a tool that measures the total dissolved solids in a liquid. This is a very handy tool to have in the aquarium hobby especially if you are using a reverse osmosis (RO) system.

Why? Although the TDS meter doesn’t determine what kinds of solids are in the water, we know from our tap water it can include a variety of things that can cause issues in our aquariums. TDS actually measure mobile charged ions these can include: minerals, salts or metals dissolved in a given volume of water, and are directly related to the purity of water and the quality of water purification systems.

TDS meters give you readings of “parts per million” or PPM. This translates to how much of the dissolved solids there are in relation to pure water. One part solid vs. 1 million parts water. To put this in perspective, 1 PPM is roughly equivalent to 5 liters of material in an Olympic-size swimming pool.

We know the more “stuff” that is in our water, the more chances we have to bring something into the aquarium we might not want or have an impact on clarity. Depending on where you live, your tapwater may contain things like chlorine, fluoride, minerals or metals — and can be up to 500 PPM. If you have old pipes or using well water, this may be higher.

The only “pure” water is distilled water but an RO system can drastically reduce the amount of impurities in water. Adding a de-ionization (DI) stage can lower this number drastically. If you are shopping for a water purification system, you’ll see plain RO systems and the combined RO/DI systems. A TDS meter is a must have to ensure your filter is doing its job and when its time to change filtration materials or the RO membrane.

Back to TDS in the aquarium…high volumes of impurities will affect water clarity and overall health of your system. Sure you may use a water treatment supplement to remove chlorine and chloramine and many also state they detoxifies heavy metals. These can be helpful, but keeping the TDS low to begin with is a better strategy.

You may see all your parameters (ammonia, nitrites, nitrates, hardness, pH, etc.) where they should be and a clean filter, but cloudy water. This could be because of a high TDS that can not only make it cloudy looking, but photosynthesis can be decreased in a planted tank and can also keep your temperature higher from the particles absorbing heat (more of an issue in summer or hot weather. But the higher TDS levels can mean there or other toxic materials in your tank like lead, copper or other things you don’t want to see.

You can get inline TDS meters for RO/DI systems, but a simple handheld “pen” can be purchased for $10-$20. These just require dipping in water for a nearly instant reading. Some may cost higher and these can include more bells and whistles like temperature compensation, may also measure other parameters or are more water resistant for the accidental drop in a tank.

Another thing to keep in mind is measuring TDS if you are purchasing or getting filtered or treated water from another source. These can include bottled water, water from a store or kiosk, or even water from your local fish store. Even though they may say they are filtered, there are always acceptable variables.

If you’re going to be in the hobby for a while, make the investment in a TDS meter, it is money well spent.