TDS (Total Dissolved Solids) is the measure of all organic and inorganic material in molecular form in your water including GH, KH, nitrate, nitrite, etc. Since they are in molecular form, TDS can more simply be seen as the substances that can not be picked up by aquarium filters. Aquarium filters can only catch what comprises TSS (Toal Suspended Solids), which are material things such as plant matter or fish waste.
High concentrations of TDS can reduce water clarity and decrease photosynthesis in aquatic plants. It can also combine with toxic compounds and heavy metals leading to an increase in water temperature. Total dissolved solids can be measured with a TDS meter which will often read in ppm (parts per million).
So what is the proper TDS level for an aquarium? Unfortunately, the answer is everyone’s least favorite:
First, it depends on your fish. Some fish are used to native streams of near zero TDS waters while others, such as African Cichlids, can live in waters of TDS between 300-400 ppm. It also depends on various substances you may add to your fish tank. Nearly anything that you add to your tank will result in an increase in TDS (water conditioner, plant fertilizer, uneaten food).
TDS = GH + KH + Nitrate + Nitrite + Chlorine + Other Molecular Compounds
The desired level of nitrites and nitrates alike is zero for both fresh and saltwater aquariums, although a nitrate level of less than 40 ppm is acceptable.
Chlorine is desired to be zero.
GH (general hardness) is the combination of calcium and magnesium ions found in water. Desired GH will depend on the fish you keep in your aquarium.
As you can see in the chart below, we know KH is typically between 120 and 180 ppm for freshwater and 180-300 ppm for saltwater.
So with all this info in mind, here is an example below this chart on how you can determine the desired Total Dissolved Solids level for your tank.
TDS = GH + KH+ Nitrate + Nitrite + Chlorine + Other Molecular Compounds
TDS = (60-75 ppm) + (70-120 ppm)+ 0 + 0 + 0 + Other Molecular Compounds
TDS should be between 130 and 195 ppm + what comes from compounds such as fish waste, plant fertilizers, algae treatment chemicals, etc.
*= Researched from various online articles and forums referenced at the end of the article
To find a base level of the Total Dissolved Solids coming from “Other Molecular Compounds”, use a TDS meter to measure the TDS of your tap water. This will give you insight as to what substances may be in your tap water. If TDS is over 300 ppm, not only will you want to consider drinking different water, but you may want to mix RO water ( 0 TDS) with your tap water to get it to a range of approximately 150-200 ppm before adding it to your aquarium.
Then, continuously monitor the TDS meter as your aquarium runs and as you add any kind of substance to your water. Once you have determined the average TDS of your aquarium from observing it for a few days, use a test strip to make sure all of the individual compounds are at their desired levels (nitrate, nitrite, gH, kH, chlorine). If all of these levels are correct, the average TDS you’ve observed should give you a good idea for your aquarium’s ideal TDS level. From there, you can keep an eye on your TDS meter and not waste test strips until you see a significant increase in TDS (approx. 10-15%). A spike in aquarium TDS would indicate that one of your tank’s levels is too high and from there, you can use a test strip to diagnose which specific level it is.
When TDS levels are too high, your fish are at an even greater risk of danger than your aquarium plants. Fish are largely comprised of water, and when they are in water with high TDS, the high TDS aquarium water wants to equalize with the lower TDS water inside the fish. The aquarium water attempts to pass through the fish until the levels equalize affecting blood pH, digestion, and its immune system, greatly increasing the risk for disease.
One of the most efficient ways to keep an eye on your TDS is with our 3 in 1 Aquarium Meter that also monitors temperature and pH!
Let me know what questions you may have and thanks so much for reading on Total Dissolved Solids.
Neon Tetra Conditions
Mixing tap with RO water