Updated: Jun 12, 2022
Freshwater Aquarium Water Chemistry
Unlike the world's oceans, bodies of freshwater are more isolated and variable in their water chemistry. Most of Earth’s coral reefs fall within the same range of water parameter values, but the saline and alkaline freshwater lakes of Africa are vastly different than the acidic blackwater rivers of the Amazon.
In this article, I will lay out the ideal water parameters for a neutral community freshwater tank, a soft water, and a hard water tank. Then I’ll discuss the differences between them. PH, general, and carbonate hardness are the only parameters that change drastically amongst the three types so I'll go into some detail on those in each section. The remaining parameters will be discussed in Part 2.
The type of tank you want to cultivate is dependent on the inhabitants within. To determine if you need a neutral, soft, or hard water tank, find out from where in the world the fish you want are from. If they are from the great lakes in Africa, then they will prefer hard water. If they are South American cichlids, they will prefer soft water. If they are livebearers, goldfish, or tetras, they will prefer more neutral water.
Hardier fish can be acclimated and kept in tanks with water parameters slightly off from what they prefer. However, the further from their ideal water parameters you keep your water, the more stress you will introduce. This beckons disease, discoloration, abnormal behavior, and general discomfort. While they may live, they will not thrive and your tank will be the lesser for it.
Neutral Freshwater Aquarium Parameters
The neutral freshwater tank, in this case, refers to a tank with neutral pH, and mid-range general hardness, and carbonate hardness. We’ll get into the specifics further on, but first, a nifty chart summarizing the ideal ranges for the most important water parameters.
6.8 - 7.4
5.5 - 9.5
4 - 8 dGH
0 - 18 dGH
4 - 7 dKH
0 - 12 dKH
74 - 80 degrees F
64 - 82 degrees F
< 0.1 ppm
< 0.1 ppm
< 5 ppm
< 0.05 ppm
0.005 - 0.05 ppm
10 ppb (3)
0.5 - 1 ppm (4)
1 - 100 ppm (5)
0 - 0.1 ppt
0 - 0.5 ppt (1)
5 - 15 ppm
5 - 15 ppm (1)
< 5 ppm (2)
5 - 2- ppm (1)
A neutral freshwater community tank should run a pH as close to 7.0 as possible. While very few natural freshwater environments measure out at precisely 7.0. This neutral number accommodates fish and invertebrates that prefer either slightly acidic (6.8) to slightly basic (7.2) water.
The total range of pH for most freshwater systems is only 5.5 to 9.0 out of a total pH scale of 0-14. While saltwater tanks are even more narrow at 7.8 to 8.2. Even though freshwater bodies of water from around the world can range from roughly 5.5 to 9.0, the fish of each location can not. For example, African lakes are freshwater at an average pH of 8.5 and blackwater rivers in the Amazon are also freshwater at an average pH of 5.8. However, even though they are both considered freshwater, cardinal tetras from South America would not survive in a freshwater African Lake.
Keeping with the middle ground of a neutral community tank, general hardness should be between 4 - 8 dGH (degrees of general hardness) on a scale of 0 - 18 dGH.
General hardness is the measurement of calcium and magnesium ions in the water column.
If you have heard of water being “hard” or “soft”, this is in reference to the concentration of calcium and magnesium, or the general hardness. Environments rich in minerals, such as the central United states and the east coast of Africa have harder water than the muddier, less rocky soils of the Amazon rainforests.
Aside from the litany of chemical relationships calcium and magnesium have with the other elements in your aquarium water, general hardness affects your fish, inverts, and plants primarily by osmoregulation. Fish that evolved in more neutral waters are not equipped to regulate the excess amount of calcium and magnesium in harder water, or the lacking amount in softer water. If the general hardness (GH) stays between 4 and 8 dGH, these neutrally inclined species will be much better off.
The relationship between general and carbonate hardness is intimate. While general hardness measures the amount of calcium and magnesium ions in the water, carbonate hardness is a measure of the amount of calcium carbonate (CaCO3) and magnesium carbonate (MgCO3) in the water.
The amount of carbonate in the water is also a reflection of the amount of CO2 and hydrogen ions, resulting in a relationship with pH as well.
Therefore, neutral tanks will have a carbonate hardness of between 4-7 dKH (degrees of carbonate hardness). Increasing or decreasing the carbonate hardness will increase or decrease your pH as well.
Converting some of the carbonate to carbon dioxide will lower the pH, while adding more carbonate will raise it. This can be done temporarily by adding CO2 gas or more permanently by using a trusted pH buffer.
Soft Water Aquarium
A soft water aquarium is defined by a more acidic pH, and lower general and carbonate hardness. Some Asian and most South American species prefer softer waters. We generally see soft water in locations where less rocky and minerally deficient soil feed into the local bodies of water.
Popular soft water species include, but are not limited to; cardinal tetras, rummy nose tetras, other South American tetras, German blue rams, discus cichlids, Corydoras catfish, angelfish, and hatchet fish.
5.5 - 6.8
5.5 - 9.5
1 - 4 dGH
0 - 18 dGH
1- 4 dKH
0 - 12 dKH
In soft water aquariums, the pH is generally lower as these softer waters also have a lower carbonate hardness, resulting in a more acidic pH. Some species like barbs, angelfish, and Corydoras can adapt to a neutral pH, but others like the discus, German blue rams, and cardinal tetras are sensitive to pH and must be kept as close to their natural value as possible.
There are many varieties of freshwater plants that enjoy softer water as well. When the water is too hard, plants can dry out from excess concentrations of minerals and salts in the water column. If you are keeping a softer water tank (even just below neutral) you’ll have your pick of aquatic vegetation to choose from.
A lower pH can also be a benefit in soft water aquariums as ammonia is less toxic in more acidic water. While this in no way means you should allow the ammonia levels to rise above .25 ppm, if by some means the ammonia levels do rise, acidic water provides a nice toxicity buffer.
At concentrations below 4 dGH, soft water species do not regulate the higher mineral content of hard water very easily. They prefer softer water, with less mineral concentration. This limits their natural ability to regulate their osmotic pressure, meaning if they are placed in neutral or hard water with a GH above 4 dGH, they will have a much harder time keeping the concentration of minerals in the water column balanced with the concentration of minerals within their tissues. This leads to higher energy usage, and much more stress than is necessary. This will ultimately lead to disease and probable death for even slightly more sensitive species.
As we know, carbonate hardness and pH are linked. However, carbonate hardness and general hardness are not directly linked. If the water is softer (lower GH), then the carbonate hardness should be kept lower too, which means the pH will also be lower.
In our aquariums, we can artificially raise the GH without raising the carbonate hardness or the pH. But there are no species that enjoy a high GH and a low KH/pH. In the wild, GH and KH are usually aligned. This is because of the tendency of the geological surroundings to release calcium, magnesium, calcium carbonate, and magnesium carbonate simultaneously.
One caveat with soft water aquariums is that pH can fluctuate more easily with a lower KH. This means, you should try to keep the pH for most soft water species at 6.0 - 6.8 while keeping the KH as close to 4 dKH as possible to maximize stability. However, pH stability can be maintained with a KH as low as 2 or even 1 dKH. The key is to not have a level below 1 dKH.
Hard Water Aquariums
Hard water aquariums are defined as having a general hardness above 8, carbonate hardness above 7, and generally, a pH higher than 7.4.
7.4 - 9.0
5.5 - 9.5
8 - 18 dGH
0 - 18 dGH
7 - 10 dKH
0 - 10 dKH
In hard water aquariums, a higher pH usually runs between 7.4 and 9.0 depending on the carbonate hardness. Some brackish water species, livebearers, and Asian and most African cichlids enjoy a higher pH. The saline and mineral rich waters they inhabit are conducive to a higher pH.
To get the most color and health out of your already vibrant African cichlids, make sure you match the pH with the lake or body of water they are from. Lake Victoria and Malawi cichlids prefer a pH of 7.4 to 8 while Tanganyikan cichlids thrive in a pH of 8.3 to 9.
The Most popular plants for hard water aquariums are the waxy leaved Anubias, and java fern, Hygrophila, and Bacopa. These plants are capable of resisting osmosis and can effectively balance the salt gradient in and outside of their tissues.
Hard water aquariums are named as such because of their elevated levels of calcium and magnesium. Mineral and salt-rich waters are usually indicative of a higher general hardness. There are plenty of commercially available buffers to raise the general hardness of your aquarium water. Some are even formulated for specific types of water (RODI vs tap), or types of tank inhabitants (planted vs. brackish, vs. hard water vs. soft water tanks).
The relationship between carbonate hardness and pH is the same in hard water, soft water, and neutral aquariums. The key difference is the elevated pH-buffering capabilities of water with high carbonate hardness. Essentially, the higher your carbonate hardness, the more stable your pH will be.
While carbonate and general hardness have little direct correlation, they do both measure forms of calcium and magnesium. Therefore, they can affect one another if carbonate hardness is broken down and releases calcium and magnesium. More importantly, naturally occurring harder water usually contains elevated carbonates as well. This is due to the surrounding geological tendency to release calcium and magnesium along with calcium carbonate and magnesium carbonate simultaneously.
Again, like with soft water, we can artificially change this naturally occurring pattern by raising or lowering the GH without affecting the KH or pH. We don’t really have any reason to do this though, as no fish lives in these altered conditions naturally.
Always be slow, careful, and intentional when experimenting with your water parameters. Trying to change your pH can also affect your carbonate hardness and vise versa. Using subpar pH altering chemicals can crash tanks pretty quickly.
What has always saved me, and a practice I will always use, is to use a half dose when changing carbonate or general hardness. For example, if the instructions say to add 1 tablespoon for every 10 gallons to change dKH by 1 and I want to increase a 10 gallon tank by 1 dKH, I will use a ½ tablespoon. Then I’ll wait 24 hours and test again. If the KH did not exceed 1 dKH then I’ll add the other ½ tablespoon.
Most liquid test kits can’t measure in the tenths of a degree of KH or GH. They can only measure 0, 1, or 2, etc., not 1.3 or 2.5. Dosing a ½ dose at a time ensures that you don’t add too much. For example, you may test your KH at 1 dKH, but it is actually 1.7 dKH. A full tablespoon dose would raise the KH too much, causing more work down the road in your attempts to lower it again. This constant fluctuation up and down as you narrow in on your target value is stressful for your aquarium’s inhabitants as well.
Stay tuned for Part 2 where I will discuss the remaining parameters in the table at the beginning of this article. These remaining parameters are usually the same for all types of freshwater aquariums, but are still of paramount importance.
Boodleshire LLC 2021
Department of Water, Western Australia
Crosby, Tina & Hill, Jeffery & Martinez, Carlson & Watson, Craig & Pounder, Deborah & Young, Roy. (2021). “On-Farm Transport of Ornamental Fish.”
Agency for Toxic Substances and Disease Registry Division of Toxicology CDC