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Mastering Freshwater Aquarium Ecosystems

Table of Contents

  1. COVER
  14. ROCKS
  15. WOOD
  16. PLANTS


Freshwater sounds simple, but it is actually the most complicated, and least understood aquatic environment by aquarists. Freshwater chemistry varies drastically from location to location. Geographical makeup of a location can affect water composition.

Rivers, streams, creeks, ponds, and lakes that are in areas where limestone is a major part of the topography tend to have high amounts of calcium, carbonate, and magnesium. The pH is usually well above neutral (7.0).

The Rio Negro in the Amazon Rainforest is an area surrounded by jungle that has a dense organic floor that helps make the water very soft, and a very low pH (4.0). In fact the water in parts of the Rio Negro are nearly as soft as distilled water.

Adding fish that are adapted to hard alkaline water to a soft water low pH system can often prove detrimental. Knowing what type of water chemistry your fish are adapted to is very important in becoming an accomplished aquarist.

Total Dissolved Solids (TDS)

Tap water, rivers, streams, creeks, lakes, and ponds always contain dissolved solids, that can include calcium, magnesium, carbonate, sodium, chloride, silica, lead, zinc, copper, iron, and many other elements. The major elements that we are most concerned with in freshwater for fish are calcium, magnesium, carbonate/bicarbonate, and sodium. All of the dissolved solids combined make up what is known as Total Dissolved Solids (TDS).

TDS is usually measured in parts per million (ppm). What that means is, if you have a TDS of 290 ppm in your tap water, 290 of the one million parts of water are dissolved solids. Pure freshwater has no hardness, no additives, and contains only H2O, which has a TDS of 0 ppm. Distilled water that can be bought at most grocery stores is as close to pure water that the average aquarist may ever want to work with in the hobby.

Tap water is the main source of water for most aquarists, and is highly variable depending on where you live, and whether the water is surface water, or groundwater (well water). Surface water is generally softer than ground water. Ground water usually picks up quite a few minerals as it passes through the earth. Even in close geographical areas the water source can vary drastically. In the area of Sacramento, California (where I live) some areas get 100% surface water all year long, which has an average TDS of less than 50 ppm (very soft water), while other areas have 100% ground water with an average of 290 ppm (very hard water). To complicate things more for freshwater hobbyist, some municipal water districts will give you surface water one part of the year and well water the other, creating drastic changes in water chemistry. Drastic changes in your tap water chemistry can cause mortality in freshwater fish within 72 hours after doing a water change. Aquatic plants can also start to die when there is a sudden change in water chemistry.

All bodies of freshwater have their own unique water chemistry. Some bodies of freshwater, like the Rio Negro in the Amazon Rain Forest have extremely soft water, less than 20 ppm TDS, and some bodies of water have extremely hard water, like Lake Tanganyika in Eastern Africa can be 300 to 460 ppm. Different species of fish, invertebrates, and plants have evolved to live in these diverse water conditions.

Trying to keep fish from one extreme in another extreme often leads to premature death. It is very important to research what the water requirements are for the aquatic life you want to keep and try your best to recreate those requirements in your system. While freshwater fish kept in the wrong condition may survive for a while, they will not thrive. In some cases, for example, moving a fish from soft water to extremely hard water can cause an osmotic shock to the fish's system, leading to death within 24 hours. Long-term exposure will often result in the fish contracting a disease, and/or have organ failure.

Unfortunately most research that is done on fish is only looking at effects of certain elements over a short period of time, 24, 48, 72, or 96 hours of exposure. Experienced aquarists are the best source of information we currently have as to the long-term effects of water conditions on fish health. Only long-term observations and comparison can tell us how chronic exposure to certain elements, or lack of exposure, at certain concentrations can affect fish health.

In the United States of America it is easy to find out what your water conditions are for your local area. Most municipal water districts have their water quality test results posted on the Internet for their residents to view at anytime. It is highly recommended that you find out what your water quality conditions are before you decide which fish you will keep. If you are not interested in trying to change the water chemistry, stick with fish that are adapted to your water conditions. If you have tested your water or know the TDS from a water quality report, and it is not the right chemistry for the fish you want to keep, there are ways you can change the water to meet the requirements.

To make soft water hard is very easy. Many aquarium product manufacturers make products to add to the water to help bring up the hardness. Some of these products are labeled for African cichlids from Lake Victoria, Lake Tanganyika, Lake Malawi, and livebearers (Mollies, Guppies, Swordtails, Variatus, Platies). These products are often called salts, but often contain more than just salt (sodium chloride).

Sodium chloride is the same chemical as table salt, rock salt, “aquarium salt”, and salt sold for recharging whole house water softeners. Sodium chloride is only one of the many elements that make up TDS, and is not usually an element we try to add in significant quantities. Generally the primary ingredients we look for in a product to increase TDS are magnesium, calcium, carbonate, and bicarbonate. Seachem is one company that does a good job creating buffers (increase or decrease the pH) and salts for freshwater aquariums that can help you recreate as close as possible the water conditions of several unique freshwater environments.

How much of a product you will use for your system will depend on the hardness of your tap water. Seachem provides a chart on many of their products that give you examples of how to mix their products, and the ratios to use, but these charts are based on you starting with distilled, reverse osmosis, or deionized water, all waters with very little or no hardness. You will need to adjust the amount you add based on what is in your source water.

If you are adventurous, you can find the raw ingredients at the grocery store, hardware store, or pool supply store. Baking soda is sodium bicarbonate, soda ash or swimming pool pH increaser is sodium carbonate, and swimming pool calcium increaser (or Dow Flake) is calcium chloride.

Whenever you add chemicals to your freshwater system, you should always first add the chemical to a container of water and then mix thoroughly until completely dissolved. Some of these chemicals can get very hot when initially mixed with water and should be stirred quickly while mixing. Pour the mix slowly into a high flow area in the aquarium. Dumping raw chemicals straight into the system can harm or kill aquatic life.


pH is the measure of whether a solution is acidic or alkaline (basic). pH is measured on a scale of 0 to 14, 7 being neutral and the pH of pure water at 77°F (25°C). A pH below 7 is considered acidic and pH over 7 is considered alkaline or basic. pH measures the concentration of hydrogen ions in the solution. A low pH represents a high concentration of hydrogen ions, and a high pH has a low concentration of hydrogen ions. Each point on the scale represents a 10 fold increase or decrease from the next point in the concentration of hydrogen ions. Chemicals that are used to raise or lower the pH are called buffers.

Most freshwater rivers, lakes, and streams fall between 4.0 and 9.2. Fish that come from water that has a pH as high as 9.2 should not be kept in water with a pH range of 4.5 to 6.8 and vise versa.

Fish that have evolved in water that has a high pH will usually not live long in water that is acidic. Fish that are kept in a pH range that is not considered normal for that species will be under physical stress which lowers their bodies immune system. Fish that are kept in the wrong pH range often have health problems. Freshwater that is soft and has a high organic load is often very acidic. The black water (tea color) of the Rio Negro, in the Amazon Rainforest of Brazil, is one example of very soft and acidic water, with a pH value of less than 4.5 much of the year. Fish that live in the Rio Negro have evolved to tolerate such a low mineral content, and are more efficient at extracting salt from the environment than most fish. Fish that come from such waters often have a limited tolerance for water that has a lot of dissolved salts. Lake Tanganyika (in Central East Africa) is at the other extreme, its water is often measured at a pH of 9.2 or higher. Fish from Lake Tanganyika are adapted to living in hard alkaline water, and often do poorly when kept in soft acidic water.

There are several commercially available pH buffers manufactured for the freshwater hobby. These buffers are marketed to raise or lower the pH, and some are formulated to set the aquarium water at a specific point. Never add more buffer than what the manufacturer recommends in a 24 hour period.

Most freshwater fish do fine in a pH between 7.0 and 8.0, but fish that come from the extremes do a lot better if you try to come close to matching the pH found in their natural habitat. pH increasers are primarily made up of sodium bicarbonate, and to a lesser extent sodium carbonate. Sodium carbonate is more common in pH increasers intended for Lake Malawi, Lake Tanganyika, and marine aquariums. It is not all that uncommon, when sodium carbonate is added to the system (especially marine aquariums), that it will temporarily cloud the water.

Drastic changes in the pH can cause a condition known as pH shock. The symptoms are associated with the fish lying on the bottom of the tank, often on its side, and abnormal heavy breathing. The condition can be reversed if the fish is returned to the original pH and given a few days to recover. If no action is taken to return the fish to its original pH, death can occur within 24 hours

Generally a safe adjustment in pH is .5 in a 24 hour period for most fish. When changing the pH with buffers, the chemical normally changes the pH quickly, and then in a 24 hour period it balances out after interacting with organics and other chemicals in the system. Check the pH 24 hours after you made the adjustment. Add additional buffer to the aquarium if the target has not been reached.

Adjusting the pH of the water can sometimes take a week or longer to reach the target range, and maintain it there. Soft water has little buffering capacity, and any change to the chemical composition can have a radical effect. Check the pH often with soft water, at least once every two weeks. You may need to check it more often, depending on your water chemistry and how stable the pH stays.

Generally harder water with a pH in the 8’s is more stable than soft water. Making hard water acidic is much more difficult. Aquarium product manufacturers make acids to bring down the pH in the aquarium, but when used the pH has a tendency to spring back up. Some of these pH decreasers are phosphate based (example: sodium biphosphate), and some are phosphate free. Phosphate can cause excessive algae growth, so it is best to try to avoid phosphate based buffers, especially if you have live plants in the aquarium.

It is easier to make your water acidic if you start with soft water. Soft water has a natural tendency to go to the acid side of the pH scale from the dissolved organics in the water and the adsorption of carbon dioxide (CO2). Some aquarist use sphagnum peat moss in their filtration system to help make the water acidic. Normally when you set up a new freshwater aquarium the pH is likely to be on the alkaline side of the pH scale. As the aquarium ages it will build up organics from fish waste, and decaying plants and algae, and most often nitrate. As the organics build up the pH will naturally drop. It can take up to six months before you will notice the organics are bringing the pH down. In an aged aquarium you may need to add alkalinity buffer to keep the pH from dropping below a desired point.

Sunlight and bright aquarium light effects pH. At nighttime, plants stop taking up carbon dioxide and use oxygen (O2), when this happens, the pH can drop. During the time the sun is shining or bright aquarium lights are on, the pH can rise several tenths of a point as the plants take up CO2 and give off oxygen. The bright light on aquarium plants can help reduce and control nitrate, making the pH more stable.

Water Hardness (General Hardness), Soft vs. Hard

Hardness is generally thought of as being the elements of calcium and magnesium, but there are many others that add to hardness. Calcium (Ca++) and magnesium (Ma++) are the major elements we normally look at as the contributors to tap water hardness. Distilled or pure water has a hardness of 0 ppm.

Aquarium test kits measure the water hardness in ppm (parts per million), or German DH (gDH). Germans have a large influence in the aquarium hobby so some test kits measure in German DH (sometimes “General Degree of Hardness or General Hardness (GH)”). The international standard for expressing hardness value is ppm or its equivalent milligrams per liter (mg/l).

The United States Geological Survey (USGS) provides general guidelines for classification of waters: 0 to 60 mg/L (milligrams per liter) (same as ppm or parts per million) of calcium carbonate is classified as soft; 61 to 120 mg/L as moderately hard; 121 to 180 mg/L as hard; and more than 180 mg/L as very hard. Parts of Southern California are reported to have greater than 1000 mg/L.

One degree of GH (gDH) equals 17.86 ppm (mg/L).

Conversion Table

ppm gDH ppm gDH ppm gDH