Understanding water hardness and buffering properties: gH, kH and TDS

If you keep an aquarium or are interested in aquarium keeping, you have probably encountered the mysterious abbreviations gH, kH, and TDS. At first glance, they may seem like complex scientific terms, but in fact, they are simply ways to measure different properties of water. Understanding these parameters is the key to creating a healthy environment for your fish and plants.

Imagine that these indicators are like vital signs for your aquarium. They tell you how soft or hard the water is, how well it resists sudden changes in acidity, and how many different substances are dissolved in it. Ignoring them is like ignoring nutrition and climate for land animals.

In this article, we will explain in simple terms what each of these parameters means, why they are so important, and how they are related to each other. You will learn how to measure these values and what to do if they exceed the normal range for your aquarium inhabitants.

What do kH, gH, TS, TDS, and TSS mean in water?

TS (total salt volume) = TDS+TSS — this is everything that is dissolved in water. Depending on the particle size distribution, we can distinguish:

TSS (volume of suspended particles)

Total content of particles larger than 2 microns. These can be fish waste, bacteria, or decaying plant tissue. This is an instrumentally unmeasurable indicator that can be replaced by the parameter “transparency/turbidity.”

Such tests are not carried out in aquariums, except by eye, but for the curious, I will briefly describe how to test water for TSS.

Water is poured through a special filter, after which the filter is rinsed with deionized water (from under the reverse osmosis filter). This is done so that the dissolved solids (TDS) do not affect the TSS readings. Then the increase in filter weight is measured.

Working as an employee at a natural aquarium gallery, I noticed that many people commented on the extreme clarity of the water in the aquariums. This can be attributed to the TSS parameter.

This parameter is important when keeping small fish and shrimp. They have delicate gills that can become clogged with TSS particles, which can lead to their death.

TSS can be regulated:

• mechanically, using powerful filtration with external filters and internal filters with synthetic filler.

• chemically, using special conditioners for cloudy water, which precipitate such large particles from the water column to the bottom. Then they must be siphoned off from the bottom.

An abundance of suspended particles can be the result of overfeeding your fish, overcrowding the aquarium (lots of fish waste and, as a result, bacteria), plant leaves dying (which means something is wrong in the aquarium), or insufficient filtration in your aquarium.

From an aquascaping perspective, an abundance of suspended particles affects the appearance of algae on leaves, as these particles can settle on plant leaves and thus become a place (substrate) for algae to grow.

The moss that many people love can die from an abundance of TSS on leaves and stems.

Based on this, it is rare to see a large number of fish (especially large ones) and an abundance of moss in professional aquascapes at the same time.

The degree of aquarium filtration is assessed not only in terms of the volume of filter media, but also in terms of the power of the flow from the filter. Thus, the presence of a large number of suspended particles may indicate insufficient flow in the aquarium. Don't forget to take care of this as well. For example, wash the filter and change the media in a timely manner.

• In the case of a heavily overgrown aquarium, it may be a good solution to install a flow pump. It does not have a filler, but only creates an additional flow in the aquarium.

• Also, the more dissolved substances in the water, the less transparent the water is, which has a strong effect on the penetration of light into the water. And this is directly related to the process of plant photosynthesis.

I think we've figured out TSS.

TDS (total dissolved solids)

gH + kH + Nitrates + Nitrites + Chlorides + Chloramines + Sulfates + Phosphates + Ammonium + Carbonates + Bicarbonates + other compounds (fertilizers, water conditioners, fish medicines).

An instrumentally measured water parameter that shows how many chemical additives are dissolved in the water.

For example, it can be used to understand how hard or soft the water is (but this is not the optimal parameter for such an analysis), how well plants consume the fertilizers added to the water.

The following categories are distinguished by the amount of dissolved salts:

• Less than 1500 mg/l (ppm) — fresh water

• 1500-5000 mg/l (ppm) — brackish water

• More than 1500 mg/l (ppm) — seawater

A particularly high TDS value may be a sign of poor water quality.

• In the case of TSS, it is often enough to install a high-quality filter, and there will be much less suspended particles in the water, or none at all.

• In the case of TDS, mechanical filtration does not work. Here, only the chemical component of aquarium filtration will help us.

Why do you need to know TDS and where is it applied?

The first and probably most important thing is the chemical purity of the source water. Almost all professional aquascapers use reverse osmosis filters to create their aquariums.

These are special filters that are installed in the water supply lines of your house or apartment, similar to ordinary household filters, with the only difference being that such water is not drinkable and is used for specialized purposes, one of which is aquariums with plants.

Reverse osmosis filters purify tap water to an almost distilled state.

The degree of water purification is controlled precisely by the TDS parameter.

A reverse osmosis filter consists of pre-filters: a mechanical cleaning filter and a carbon filter, as well as a membrane, which is the main element of a reverse osmosis filter.

Pre-filters serve as protection for the membrane to increase its service life.

With a TDS of ~300 from the tap, a carbon filter cartridge is used up in a week, a mechanical cleaning cartridge in two weeks, but the condition of the membrane can only be assessed by TDS.

A new filter purifies water from ~300 TDS to ~4. When the TDS value of the water after the filter (usually measured at the membrane outlet) begins to rise even with new filters, it means that it is time to replace the membrane with a new one.

Usually, filters with deionizing resin are installed after the membrane. It “finishes off” the residual TDS after the membrane from 3-4 to 0 ppm.

I recommend using this type of water (0 ppm) in plant aquariums, because if we leave the TDS after osmosis at a value other than zero, we do not know what remains in this water, since we have seen earlier that TDS consists of a large number of different components.

The condition of ion exchange resins is also monitored using TDS. Thus, when using water after reverse osmosis, we monitor TDS twice.

After the membrane filter and after ion exchange resins.

SpectraPure has anticipated the need to measure TDS frequently, so it has built electronic TDS meters into its filters. Some of the company's filters even have two.

Water hardness and buffering properties are not just a set of incomprehensible numbers, but important characteristics that affect the taste of water, the operation of household appliances, and even health. When we talk about gH, kH, and TDS, we are talking about how much dissolved salt is in the water, how resistant it is to changes in acidity, and what substances are present in it. It's not that difficult to understand: gH shows the total hardness, kH shows the water's resistance to acidification, and TDS shows the total amount of dissolved substances. Understanding these parameters helps you choose the right filter, protect your kettle from limescale, or simply drink more comfortable water.

What TDS water is best to use in a planted aquarium?

It depends on the fertilizers you plan to use in your aquarium and the fish and plants you plan to keep.

For example, fish from Lake Tanganyika live in nature in water with a TDS of ~400 ppm, while fish from the Amazon River (such as the well-known neon tetras, Paracheirodon axelrodi) live in water with a TDS close to 0. Thus, TDS (here it can be equated to water hardness gH — what this is will be explained below) is one of the determining factors for future aquarium inhabitants.

Since reverse osmosis filters purify the water as much as possible, leaving only H2O molecules, this means that nothing useful for plants remains in the water either. Therefore, it is necessary to consider adding fertilizers with the widest range of macro- and microelements (range, not concentration).

There are two broad categories of fertilizers: those that are suitable for use in pure osmosis and those that are not.

The following companies' fertilizers are suitable for osmosis:

The rest of the fertilizers are marked “worth trying.”

I have never been shy about telling my customers, “I don't know, you have to try it,” because aquarium keeping is a hobby without strict rules, and all advice is only a recommendation.

I recommend using fertilizers from other manufacturers with water that has a TDS of ~100 ppm.

How to prepare water with the required TDS?

• Restore water with gH+kH remineralizers

• Or mix osmotic water with tap water.

Many people use the second method because it is cheaper: you can buy simpler fertilizers (which are not used in osmosis) and you don't need to buy remineralizers.

I recommend the first method. It allows you to fully control the parameters of the source water.

What TDS does your aquarium need?

Make a list of the aquarium inhabitants:

Find out what the optimal TDS parameter is for each of them (what it is in nature — it may take some time to find this information) and choose the average value. You may have to give up some of the selected population.

If you plan to keep shrimp, pay special attention to the TDS value of the water you prepare. The minimum TDS value for keeping shrimp is ~100. Of course, there are cases of successful shrimp keeping in aquarium water with a lower salt content, but this is the exception rather than the rule.

TDS is an instrumentally measurable parameter of water, so you can use an electronic TDS meter to measure this parameter in your aquarium (and, for example, to measure this parameter in your tap water or after a household filter, as water with a high TDS is also harmful to humans).

Ideally, at the beginning and end of the week (before changing the water), your aquarium should have the same value for this parameter. This will mean that fertilizers are consumed in full, do not accumulate in the aquarium, and therefore algae have only a minimal chance of growth.

Fish bodies contain water and are found in water. Fish cells separate these two types of water, but they are semi-permeable. Water with a higher concentration (more dense) will try to pass into water with a lower concentration (less dense) until they become equal.

If fish could not somehow control this natural flow, they would either quickly become dehydrated or explode. But fish are able to control this through osmoregulation, a complex series of chemical processes.

The kidneys primarily work to eliminate excess water, but another function is to conserve and reabsorb essential salts. Both processes work to maintain a specific balance of salt and water.

Thus, high osmotic pressure (caused by elevated TDS levels outside the fish's natural habitat) will overload the fish with excess water and overload the kidneys, while low osmotic pressure (caused by TDS levels below the fish's natural habitat) will deprive the fish of water, causing dehydration.

TDS also affects the rate at which water penetrates fish through osmosis. “Clean” water passes through fish cells very quickly, while water with some TDS will move more slowly. Fish use their kidneys to pump this water out.

The kidneys of fish found in hard water do not have to work very hard. Soft water fish are built to live in water that they absorb quickly to remove toxins.

A small tetra will urinate more than three times its weight every day.

But the higher the TDS, the harder it is for fish to do this, so toxins remain in their bodies longer.

They affect their physiology, causing stress, and this will inevitably lead to a reduction in lifespan depending on the species and the difference between the TDS of the natural habitat and the TDS in the aquarium.

The best TDS meters

Personally tested TDS meters that can be ordered with free shipping on Aliexpress. Personally, I use a device from Xiaomi.

The best and most affordable TDS meter from Xiaomi for only 600 rubles

Professional 5-in-1 digital water tester. Cost: 2,000 rubles.

The most advanced 7-in-1 water tester. Cost: 2,700 rubles.

The relationship between TDS and gH (total dissolved solids)

As we already know, TDS includes many different salts, and by measuring only this indicator, we do not have enough information to understand what kind of water we have. Therefore, it is convenient to measure the indicators included in TDS separately (of course, if necessary).

gH is one of the main components of TDS — it is a combination of calcium and magnesium salts. They are usually represented by sulfates and chlorides. This is important when using osmosis water remineralizers. Make sure that the salts are represented by sulfates.

When talking about hard or soft water, we are referring specifically to the gH indicator. Therefore, the hardness of water containing only remineralizer salts (for example, freshly prepared water) can be measured with a TDS meter, rather than a drop test.

The role of calcium and magnesium in a planted aquarium

Calcium is one of the most important elements for a planted aquarium, playing a very important role in the metabolism of aquatic plants. Calcium activates enzymes, is a structuring element for cell walls, influences water movement in cells, and is an essential element for cell growth and division.

Calcium deficiency leads to slower growth of stems and roots.

Symptoms range from distorted, twisted young leaves to black spots on leaves, and yellowing of the leaves at the edges is also possible.

Since calcium is not transferred to new leaves, deficiency symptoms usually appear on them first, although not always. Pale spots or general whitening of the leaf (yellowing) can also be signs of calcium deficiency.

Calcium deficiency leads to root dysfunction and can cause iron poisoning.

• Magnesium is also a macronutrient in a planted aquarium. Magnesium helps improve flowering, enhances green color, and can even help plants grow denser. It accounts for 0.2% of the dry matter of a plant, and in some plants, the concentration of magnesium in the tissues is comparable to that of phosphorus, the main nutrient.

• Magnesium is one of the components of chlorophyll. In addition, it activates the action of many enzymes. Some sources classify magnesium as a mobile element (from one part of the plant to another), so the symptoms of magnesium deficiency are primarily observed in older leaves. However, the magnesium content itself is not as important as the Ca:Mg ratio.

According to many aquarists, including Diana Walstadt, the optimal Ca:Mg ratio is 4:1, but no less than Ca (20-30) — (5-8). Despite this, manufacturers may use other Ca:Mg ratios in their fertilizers (not remineralizers), up to 1:1.

However, an aquarist from Argentina, after conducting numerous experiments with fertilizers and selecting the optimal water parameters for aquariums with plants, claims that the ratio should not be 4:1, but rather 1:4.

I personally experimented with increasing the magnesium level in the water, adding enough to make it four times higher than the calcium level, but I did not see any significant differences in the results. Perhaps it takes much longer than I gave it to rebuild the aquarium to the new Ca:Mg ratio.

Interveinal (intercellular) chlorosis, sometimes in the form of spots, begins at the tips and edges, then spreads to the center without affecting the central vein of the leaf. Over time, the symptoms of magnesium deficiency spread to young leaves.

Other sources suggest, on the contrary, that symptoms begin on young and middle leaves. Leaf areas affected by chlorosis may change color from yellow to brown (or even purple and red).

In the case of a serious magnesium deficiency, necrosis and active leaf fall may occur.

Sometimes leaf fall occurs without any prior symptoms.

Some sources claim that young leaves can also bend and decrease in size.

Magnesium deficiency can be caused by insufficient separate application or high levels of calcium, potassium, or sodium. Conversely, an excess of potassium or calcium blocks magnesium uptake, causing a deficiency in plants.

From personal experience: for the last three years, I have been working at the @forms_of_life_moscow aquarium gallery, where I used pure osmosis with TDS ~0 and ADA fertilizers, without additional remineralization of the source water. I did not observe the described problems with calcium or magnesium deficiency, since these fertilizers contain both calcium and magnesium. Thus, when using the most complex fertilizers (ADA, Tropica, Prodibio), it is not necessary to add calcium and magnesium, as they are most likely already included in the composition.

Carbonate hardness dKH

Now, a few words about the most complex parameter of water—temporary hardness (carbonate hardness).

I will not write much about this parameter here, as a detailed description of it would require a separate article.

dKh is the total content of carbonate and bicarbonate ions in water. And dKh can only be called hardness if it is calcium carbonate or magnesium carbonate, i.e., when carbonates are added, calcium or magnesium is also added, which are released into the water when they react with an acidic environment.

This parameter is correctly referred to as “alkalinity.” It is also called a buffer. Strangely enough, this is not related to the pH of the water, which indicates whether the environment in the aquarium is alkaline or acidic.

Alkalinity is the ability of water to maintain a stable pH (to prevent pH fluctuations — this is why dKh is such an important parameter). This is important to consider when supplying carbon dioxide intensively.

Active saturation with carbon dioxide can lead to a sharp drop in pH (a shift to an acidic environment). To prevent this from happening, a carbonate buffer dKh must be added to the osmotic water. The higher the dKh, the more stable the pH, but the more carbon dioxide CO2 must be supplied to shift its value by the same amount (which is why dKh should not be high).

Above, I wrote that water can be remineralized using gH and kH salts, or gH alone. How do you know which remineralizer to use?

If you use aquasoil substrate (e.g., ADA Amazonia), it already contains a buffer, but not an alkaline one in the form of carbonates, but an acidic one in the form of humic acids, due to which the pH level will always decrease and remain at a value of ~6.7 (the optimal pH level in an aquarium is 6.6-6.8) in an aquarium with such substrate.

Thus, there is no need to add a buffer to aquariums with Soil-type substrate, and when using complex fertilizers containing calcium and magnesium, there is also no need to add gH.

It has been observed that in natural waters, the dKh level usually does not exceed the dGh level, but gH can be 1.5-2-3 times higher than dKh.

Understanding water hardness and mineralization indicators may seem difficult, but it is the key to understanding the needs of your aquarium or even your own health. Now you know that gH is total hardness, which is important for fish and plants, kH is carbonate hardness, which stabilizes pH, and TDS shows the total amount of dissolved substances.

Don't strive for some “ideal” numbers for everyone. The main thing is to understand what parameters are needed for your specific aquarium inhabitants or correspond to your household goals. Regular measurements will help you notice changes in time and take action.

We hope this information has made the water a little “clearer” for you. With this basic knowledge, you will be able to take a conscious approach to caring for your aquarium, choosing filters, or simply better understand what is written on a bottle of drinking water. Good luck with your experiments and observations!