Tropical Fish Keeping banner
1 - 4 of 4 Posts

22,557 Posts
Discussion Starter · #1 · (Edited)
Total Solids basically refers to organic and inorganic matter that is either suspended or dissolved in the aquarium water.

Total Suspended Solids (TSS) refers to the amount of solid waste, decaying fish and plant matter, etc. that can be captured and held by a filter.

Total Dissolved Solids (TDS) is a measure of the combined content of all organic and inorganic substances contained in the water in molecular, ionized or micro-granular (colloidal sol) suspended form. Generally the operational definition is that the solids must be small enough to survive filtration through a sieve the size of two micrometer.

Fresh water by definition contains no more than 1500 mg/l of TDS. Brackish water contains 1500-5000 mg/l, and marine (salt) water has more than 5000 mg/l of TDS. Note that mg/l is basically equal to parts per million (ppm), and also that this is not suggesting a level of 1500 ppm in an aquarium; these are just the approximate figures for the three categories.

TDS is connected to GH (general hardness) because like GH, TDS includes the calcium, magnesium and other “hard” mineral ions; these ions are what we measure with our GH test kits. But water hardness correctly considered is more than this; both GH and KH can affect hardness and TDS levels; however, the reverse is not necessarily true. Aquarium water can have a high TDS level but a low GH and KH (Jensen, 2009). The TDS for instance also includes sodium (salt) ions, chemical substances, etc. which are not reflected in the GH.

TDS is basically everything dissolved in the water: chlorine, chloramine, ammonia, phosphate, salt, hard minerals (GH), bicarbonates (KH), etc. And almost every substance added to the water will increase TDS: water conditioner, fish foods, plant fertilizers, calcareous substances, medications, water adjustment products, etc.

The Effect of TSS and TDS in the Aquarium and on Fish

As the above definition indicates, filtration via aquarium filters will (or should) remove the TSS but will not remove any TDS from the water [see later on carbon]. While live plants can use some of them, only a partial water change effectively removes the TDS that naturally increase within the aquarium.

High concentrations of TDS may reduce water clarity, contribute to a decrease in photosynthesis, combine with toxic compounds and heavy metals, and lead to an increase in water temperature (Jensen, 2009).

But the greatest impact, and the one that is not usually apparent to the aquarist until things are too far gone, is the impact on fish. Hard water fish--as we term those species such as livebearers, rift lake cichlids, and some of the atherinids, cyprinids and catfish—can withstand higher TDS than soft water fish. The TDS in Lake Tanganyika is around 400 ppm. Compare this to the near-zero TDS in many Amazonian streams.

Fish live in water, and their bodies contain water; the fish’s cells separate these two waters, but the cells are semi-permeable, which means the cell will permit the movement of water and certain non-polar molecules to pass through either way (called osmosis) but will prevent the passage of larger or charged molecules. The way the water moves is determined by the difference in concentrations between the two waters: water of higher concentration (more dense) will attempt to pass through to the water of lower concentration (less dense) until the two are equal. If the fish could not somehow control this natural flow, it would either rapidly dehydrate or explode. But fish are able to control this through osmoregulation, a complex series of chemical processes. The water moving in or out of the fish’s body will likely have a different pH, so another set of processes controls the function of regulating the pH of the fish’s blood (Muha, 2005). Both of these processes also affect the ability of the blood to carry oxygen, and this impacts many other functions including digestion, the immune system, and so on.

The kidneys primarily work to eliminate excess water, but another function is the conservation and reabsorption of essential salts. Both processes work to maintain a specific salt/water balance. This osmoregulation of bodily fluids requires a great amount of metabolic energy. So a high osmotic pressure (caused by elevated levels of TDS outside the fish’s natural range) will overwhelm the fish with excess water and overwork the kidneys, while a low osmotic pressure (caused by TDS levels below those of the fish’s natural range) will deprive the fish of the water needed for the kidney functions (Evans, 2004).

The TDS also affects how fast water moves into the fish via osmosis. “Pure” water would pass through the fish's cells very quickly, while water with some TDS would move more slowly. Fish use their kidneys to pump this water out. The kidneys of fish that occur in hard water don't have to work very hard. Soft water fish are built by their natural evolution to live in water that they rapidly take in to flush out toxins. A small tetra will urinate more than three times its body weight every day. But the higher the TDS, the harder it is for the fish to do this, so the toxins remain longer in their bodies affecting their physiology, causing stress, and this will inevitably lead to a shorter lifespan depending upon species and levels. This may not be evident to us until the fish just dies, for no “apparent” reason. Such fish actually become dehydrated, and most suffer kidney problems. [Geisler (1987) covers the kidney deterioration in cardinal tetra due to hard water calcium with scientific data on the fish’s lifespan directly determined by the TDS of the water.]

TDS also directly and significantly impact on osmoregulation occurring in the gills. When the TDS cause a change in the osmotic pressure, the red blood cells can change shape; a low osmotic pressure will deplete the red blood cells of water, causing them to collapse, and a high osmotic pressure will inundate them with water, causing the cells to expand. Both results will seriously impact respiration.

[Continued in Part Two, next post]

22,557 Posts
Discussion Starter · #2 ·
Total Solids, part two

How to Measure TDS

The GH is one indicator of some TDS, but only the “hard” mineral ions as mentioned previously. A TDS meter is available, but costly. Measuring the conductivity [electrical currents passed through the water] will indicate TDS; this is how most ichthyologists, recognizing the tremedous importance of TDS, measure total hardness in tropical watercourses.

For the home aquarist, if water parameters close to those of the fish’s natural habitat can be approximated, this measuring should be unnecessary. But the TDS meter is gaining interest, especially with those who understand that this test is a more reliable indicator of water quality than both the GH and pH tests [see Jensen, 2009 for more information].

How to Deal with TDS

Partial water changes; for most aquarists, this will be sufficient, at least to deal with TDS that build up in the aquarium—and over time, continually and constantly, the TDS are increasing day by day. They enter the aquarium via fish food, water conditioners, plant fertilizers, medications, and any substance that treats water in some way. Use no more than what is essential, and avoid any that are not. Obviously this becomes even more important if the TDS of the source water are elevated to begin with.

Never add aquarium salt to a freshwater aquarium except as a specific treatment and then only if suited to the fish species; this can send the TDS soaring, further stressing the already stressed fish [more here: ].

Brita filters will apparently remove some TDS [I’ve no data on how effective this is]. Carbon filtration is believed by some aquarists to remove some of the TDS, but in planted tanks this is also going to remove much-needed nutrients like DOC (dissolved organic carbon) which are essential to maintain the plants and they can help with TDS. RO units will remove TDS. Rainwater is usually safe to use. Never use bottled drinking water; this likely has more TDS than most tap water.

The use of wood, dried leaves and peat also lowers TDS along with the pH. This also works to keep parasites and (detrimental) bacterial populations low. Some of the cleanest, healthiest and purest streams in the world are the blackwater watercourses in the tropics.

For most of us, maintaining fish suited to our water parameters, or providing by natural methods water parameters close to those preferred by the species, will work fine.

When acclimating new fish, or moving fish from one aquarium to another, TDS [which includes GH] is probably even more important than pH. What is often termed “pH shock” is now being seen more as “TDS shock.” Fish have been shown to withstand fairly significant pH shifts when the TDS was low in both waters (Jensen, 2009), something I can attest too from my own experience. It is the TDS, not the pH, that shocks them. The effects of shock can be offset by slowing mixing the waters. And this can be important between your own tanks too, as TDS is unique to each aquarium.

To end, here is some advice from Dr. Neale Monks concerning domestic water softeners.

“Domestic water softeners do not produce soft water in the sense that aquarists mean. What domestic water softeners do is remove the temporary hardness (such as carbonates) that potentially furs up pipes and heaters by replacing it with permanent hardness (such as chlorides) that does not. While you can pass this softened water through a reverse-osmosis filter to remove the permanent hardness as well, until you have done so, you shouldn't consider the softened water as being suitable for soft water fish.

In fact, aquarists are divided on whether the resulting softened water is safe for keeping fish at all. The odd balance of minerals in softened water is not typical of any of the environments from which tropical fish are collected. While the chloride levels are much higher than those soft water fish are adapted to, the levels of carbonate hardness are too low for the health of hard water fishes like Rift Valley cichlids, goldfish, and livebearers. So the safe approach is not to use it in any aquarium, and instead draw water from the unsoftened drinking water source in the kitchen.”


Evans, Mark (2004), “The Ins & Outs of Osmosis,” Tropical Fish Hobbyist, February 2004, pp. 76-84.

Geisler, Rolf and Sergio R. Annibal (1987), “Ecology of the Cardinal Tetra, Paracheirodon axelrodi (Pisces, Characoidea), in the River Basin of the Rio *****, Brazil, as well as Breeding-related Factors,” Tropical Fish Hobbyist, Volume XXXV, No. 12 (August 1987), pp. 66-87.

Jensen, Niels (2009), “The Importance of Total Dissolved Solids in the Freshwater Aquarium,” as reposted on PlecoPlanet at

Loiselle, Paul, “Aquarium Water That is Too Hard,” on FishChannel at:

Monks, Neale (1), “A Practical Approach to Freshwater Aquarium Water Chemistry,” Wet Web Media.

Muha, Laura (2005), “Stress” in “The Skeptical Fishkeeper” column, Tropical Fish Hobbyist, December 2005.

“Total Dissolved Solids” entry in Wikipedia online at

Weitzman, Stanley H., Lisa Palmer, Naercio A. Menezes and John R. Burns (1996), "Maintaining Environmental Conditions Suitable for Tropical and Subtropical Forest-adapted Fishes, Especially the Species of Mimagoniates," Tropical Fish Hobbyist, Volume 44, No. 11, June 1996 (Part One), pp. 184-194 and July 1996 (Part Two), pp. 196-201.
1 - 4 of 4 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.