Freshwater Chemistry

Freshwater aquarium chemistry is one of few things I consider to be overemphasized in the hobby. It is covered in every book, beginner’s or advanced. It is covered in every forum, every store, every aspect of the hobby. However, it is not as important as most of these sources make it seem. This leads many people to become so obsessed with their test results that they forget to just look at the big picture. This can end up causing problems, not preventing or fixing them.

In freshwater aquariums, the most common tests are pH, ammonia, nitrite, nitrate, and hardness. These all need to be understood before an aquarist can understand what actually needs to be tested, altered, and how it can be safely managed.


PH is the measurement of how acidic or alkaline water is. It is measured on a scale of 0 to 14. 7.0 is considered neutral. Below 7.0 is acidic and the closer to 0, the more acidic. Above 7.0 is alkaline, the closer to 14, the more alkaline. The scale is logarithmic, as opposed to linear, and therefore the difference between 5.9 and 6.0 is the same as the difference between 6.0 and 7.0.

The pH that aquarium fish are native to can vary drastically. The Amazon River can be extremely acidic and dark with tannins, which is called blackwater, and have a pH of less than 6.0. The land-locked lakes in the Great Rift Valley in Africa (such as Lake Malawi and Lake Tanganyika) can be 9.0. These lakes are fed by rivers but have no drain. Because of this, all the minerals that end up in them stay in them. As they have built up, they have altered the chemistry of these lakes to extremes.

Most fish in the hobby are raised on farms in water much closer to neutral. This means that even if the fish are from waters that are naturally extreme in pH, their parents bred in close to neutral water and they grew up in it (probably for many generations). Because of this, there is a growing trend in the hobby to not alter pH in any way and simply focus on overall water quality. Most aquarists have found it is much better to keep a good water change schedule that will provide high water quality with a stable pH rather than adding products that increase or decrease pH. One major issue with altering pH is that if the product works at all, it is usually short-term. This means that the aquarist tests the pH and sees that it is 7.5, too high for his tetras, so he adds a product that lowers the pH. He tests it again and sure enough, it is down to 6.5 where he wants it. The next day, he tests again and sure enough, it is back up to 7.5, so he lowers it again. In some cases, aquarists add even more pH-altering product than recommended and drop it even further, only to have it go back up again. This roller coaster pH is actually much, much more stressful for fish than simply being at a stable pH of 7.5.

There are few safe ways to alter pH, and few situations where it is warranted. Although fish can live and breed in a different pH than they would have in nature, they are native to waters different than the farms they were raised on. Fish evolved in and adapted to water of a specific pH and many aquarists have found that the closer their aquariums are to the natural pH of the fish they are keeping, the better the fish do. It may not be extreme, but the fish will have better colors, be more resilient to stress, have fewer health problems overall, and breed even better.

An aquarist should test the pH at least once, ideally before stocking any fish. This can allow you to select fish that are native to water similar to the natural pH of your tap water and aquarium. In my experience, although fish can live and even breed in a pH unlike what they would have in nature, an aquarist is more likely to have problems if they choose to keep fish in water that is the opposite of what they would have in nature. This may not mean they will die quickly, but may not live as long, grow as large, be as colorful, etc. Because of this, it is usually better to choose fish that are from water similar to your tap water. So if you have hard water, you may want to avoid things such as tetras and choose to keep things such as livebearers, barbs, danios, etc. instead.

Most tap water, whether from the city or a well, is neutralized. This means that it is treated to maintain a pH of around 7.0 while it runs through the pipes. This keeps the water from corroding the pipes or leaving deposits that can eventually clog them. However, this neutral pH usually only lasts a day or two. If the water is left out (or in an aquarium), it will return to its natural chemistry. Because of this, it is important to test the water in the aquarium after it has been running for at least a couple of days and not fresh out of the tap.


Hardness is a very frequently misunderstood aspect of water chemistry. Part of the confusion is because there are actually two types of hardness, general hardness (GH) and carbonate hardness (KH).

It is important to note that the term ‘alkaline’ refers to both a higher pH as well as carbonate hardness (KH). This is actually not too misleading since high alkalinity (KH hardness) will create a high pH (alkaline water).

GH is a measure of calcium and magnesium ions in the water. This type of hardness is what is usually referred to when discussing hardness in relation to tap water because it effects how well soap lathers, mineral deposits inside pipes, etc. However, KH is of much more importance in aquariums because it is the main thing controlling the pH.

KH is a measure of carbonates and bicarbonates, both of which provide buffering capacity which is defined as the ability to resist a change in pH. A high KH means there are a lot of carbonates and bicarbonates dissolved in the water. This will create a high, stable pH that is very resistant to change. This is why the example in the pH section above kept going back up to 7.5 so quickly. This is why I refer to them together as pH/KH because they are effectively the same thing.

What this means for aquarists is that if they are having an issue with pH, they need to address KH because by doing so they will alter what controls the pH in the first place. If the pH needs to be increased, then the KH needs to be increased. This is actually relatively easy and safe and can be done in many ways. Some people will use crushed coral as a substrate and use reef type rock in an aquarium that will house African rift lake cichlids which are native to the extremely alkaline lakes. An even easier and more effective method is to place crushed coral in media bags in the filter exactly as if it were carbon. This provides better flow than it would receive sitting on the bottom of the tank, which at best will have a fraction of the flow in the filter. The crushed coral will slowly dissolve, release carbonates and bicarbonates into the water column, and increase the KH and therefore pH. It should be replaced regularly just like carbon since a biofilm of nitrifying bacteria can develop on the bagged crushed coral which can actually prohibit the crushed coral from dissolving. Please read the Filter Media Types Guide articles for more details on this.

The other common method of increasing pH/KH is to add products specifically designed to buffer pH to higher levels. These are usually designed for African rift lake cichlids or other fish from alkaline water such as goldfish. Some products simply increase the pH while others will buffer it to a specific pH (such as 7.5 or 8.2). There are even products made for specific lakes. One made for cichlids from Lake Tanganyika will buffer the pH as high as 9.0. Others made for Lake Malawi or Lake Victoria will buffer to around 8.2. Good old-fashioned baking soda can also be used to safely raise the KH. It may take a little trial and error to figure out the correct dosing though, so start with less than you think you need and test often in the beginning and then occasionally to make sure it is correct. If you add buffers, baking soda, etc., it is best to do it each week during the water change. Dose the proper amount when you begin filling (add it right after you start filling) so it can treat the incoming water). This keeps the chemistry stable in the tank.

If the pH/KH needs to be decreased, then it can be much harder to do and is more dangerous. The problem is that if the water naturally has a higher pH/KH then either it must be diluted or used up. A lower pH/KH also makes the water very unstable and it can take much less to drastically drop the pH, referred to as a crash, and possibly shock or even kill the fish. There are buffers made to lower the pH/KH but in most cases, they do not provide a stable pH, and usually not for very long at all. Using peat is one method used to lower pH/KH. It is usually used as a filter media in the filter just like the crushed coral mentioned above. The way peat works is that it releases tannins and tannic acid. These discolor the water to a merky reddish brown and the acid uses up the buffering capacity of the KH, effectively decreasing the KH and therefore pH. However, its effectiveness is greatly reduced and lifespan shortened the higher the natural pH/KH is. The discoloration is also a major eyesore to almost any aquarist. Although the fish may actually prefer it, if the visibility in the aquarium is low and the aquarist can’t actually see the fish very well, it makes for a tank that is less enjoyable. Because of this, the peat method is usually reserved for breeding where the needs of the fish are more important the overall look of the tank.

The safest way to lower pH/KH is to dilute the tap water with reverse osmosis (RO) water. RO water is water that is filtered very well so that there is little to nothing in it. Since pure water does not conduct electricity at all, the conductivity, or total dissolved solids (TDS), is used to measure the purity of RO water. Ideally it has a TDS of 0 (there is absolutely nothing in it) but few RO systems can achieve this, at least not for very long. Most RO systems made for drinking water are far from adequate since they only remove some of the TDS. An RO system made for aquarium use will get the water down to 0, or close to it. By using RO water mixed with tap water, the KH of the tap water is lowered. So if an aquarist uses half tap and half RO water, then the KH of the tap will be cut in half. This will very effectively reduce the pH/KH while still preserving enough of what is naturally in the tap water to allow it to remain much more stable than using pure RO water. Using pure RO water is very risky since there is nothing in it to help keep the pH stable. There are products that are supposed to restore some of this beneficial chemistry. However, in my experience, none of them do it very well at all, it takes some trial and error to figure out what all is needed and its exact dosing, and still allow for an unstable pH that can easily crash. Why use pure RO water and spend money on products to help make it a little more stable when your tap water has everything that is needed already in it for free?

Ammonia, Nitrite, and Nitrate

Ammonia, nitrite, and nitrate are all part of the nitrogen cycle of every aquarium. Fish release ammonia as a nitrogen waste product. There are also bacteria in the aquarium that break down fish poop, extra food, and anything else that is rotting in the tank. These also give off ammonia. Ammonia is consumed by a particular type of bacteria that uses ammonia and gives off nitrite. Another type of bacteria consumes nitrite and gives off nitrate. The bacteria that consume ammonia and nitrite are called nitrifying or beneficial bacteria. They need a lot of oxygen so they develop colonies in the filter where the flow is very high. This flow not only provides the oxygen but their food (ammonia and nitrite) as well.

Cycling and nitrate are both too important to briefly cover here. Please see the articles Cycling and Understanding the Good Bacteria and Water Changes and Water Quality for full details.

Test Kits

There are a lot of test kits on the market and some are much better than others. Generally, test strips are not very reliable and should not be relied on for accurate measurements. Liquid test kits are much better. Fortunately, even the low-end liquid test kits are very good. By far, the most popular liquid test kits are the ones made by API. If your tank is already cycled and you don’t plan on adding another tank any time soon, you can skip the master test kit. Everyone should have at least a nitrate test kit so they can ensure their weekly water changes are large enough. If you have freshwater shrimp or are working with RO water in any way, you will need a TDS meter and a GH/KH test kit in addition to the typical freshwater test kits.

If an aquarist only has one test kit, it should be for nitrate.


The most important thing to test is nitrate because it is the best reflection of overall water quality. If the nitrate is maintained at low levels the other bad chemicals that build up are likely to be low as well. Ammonia and nitrite should never be an issue once the tank is cycled. Hardness should only be tested if the pH needs to be adjusted. The pH should be tested once so that the aquarist knows the natural chemistry of his water and can choose fish that better suit it and it should only be altered in specific circumstances and using safe methods.

Other Articles You May Be Interested In

Cycling and Understanding the Good Bacteria

Water Changes and Water Quality

Filter Media Types Guide

Choosing Fish

Stocking an Aquarium

Algae Control

Live Plants

Sand as a Superior Substrate

Heaters – The Ticking Timebomb in Every Tank