Water Chemistry
The Basics of Water Chemistry and Why
Source: https://users.cs.duke.edu/~narten/faq/chemistry.html
Water in nature contains dissolved salts, buffers, nutrients, etc. and fish and plants have evolved over the millennia to adapt to the water conditions in their native habitats. Moving them from their natural environments to significantly different environments can cause survival problems.
Note: If you are new to the aquarist hobby, try taking the easy approach of selecting fish whose needs match the qualities of their normal tap water. Once you become comfortable taking care of your “starter” fish, you can move to being an advanced aquarist and you can add more varieties of fish and change the water characteristics to match the fish’s needs – just keep in mind that doing so can often be more difficult than it first appears. Regardless of your level of expertise, you still need to know enough about water chemistry to make sure that the water in your aquarium has the right properties for the fish you are keeping.
Water has four measurable properties commonly used to characterize its chemistry – Acidity (pH), Buffering, General Hardness (GH) and Salinity. Let’s take a look at each of them:
pH refers to water being either an acid, base, or neither (neutral). A pH of 7 is said to be neutral, pH’s below 7 are “acidic” and pH’s above 7 are “basic” or “alkaline”. Like the Richter scale used to measure earthquakes, the pH scale is logarithmic. A pH of 5.5 is 10 times more acidic than water at a pH of 6.5. Thus, changing the pH by a small amount (suddenly) is more of a chemical change (and more stressful to fish!) than might first appear.
To an aquarist, two aspects of pH are important:
- Rapid changes in pH are stressful to fish and should be avoided. Changing the pH by more than .3 units per day is known to stress fish. Therefore, you want the pH of your aquarium to remain constant and stable over the long haul.
- Fish have adapted to thrive in a certain (sometimes narrow) pH range and it’s best to stay with that pH range. The good news is that most fish can adjust to a pH somewhat outside of their optimal ranges. If your water’s pH is naturally within the range of 6.5 to 7.5, you will be able to keep most species of fish without any problems and there is probably no need to adjust it. This is especially important if you plan to keep multiple species of fish in the same aquarium.
Buffering capacity refers to water’s ability to keep the pH stable as acids or bases are added. pH and buffering capacity are intertwined with one another; although one might think that adding equal volumes of an acid and neutral water would result in a pH halfway in between, this rarely happens in practice. If the water has sufficient buffering capacity, the buffering capacity can absorb and neutralize the added acid without significantly changing the pH. A buffer acts sort of like a large sponge. As more acid is added, the “sponge” absorbs the acid without changing the pH much. The “sponge’s” capacity is limited, however; once the buffering capacity is used up, pH changes more rapidly as acids are added.
In freshwater aquariums, most of water’s buffering capacity is due to carbonates and bicarbonates. As a result, the terms “carbonate hardness” (KH), “alkalinity” and “buffering capacity” are used interchangeably. While technically they’re not the same things, they are equivalent in practice, as far as fishkeeping is concerned.
Note: the term “alkalinity” should not be confused with the term “alkaline”. Alkalinity refers to buffering, while akaline refers to a solution that is a base (e.g., pH > 7).
By now, you’re wondering “How much buffering does my aquarium need?” Most aquarium buffering capacity test kits actually measure KH. The larger the KH, the more resistant to pH changes your water will be. An aquarium’s KH should be high enough to prevent large pH swings in your aquarium over time. If your KH is below roughly 4.5 dH, you should pay special attention to your aquarium’s pH (e.g, test weekly, until you get a feel for how stable the pH is). This is ESPECIALLY important if you neglect to do frequent partial water changes. In particular, the nitrification cycle creates a tendency for an established aquarium’s pH to decrease over time. The exact amount of pH change depends on the quantity and rate of nitrates produced, as well as the KH. If your pH drops more than roughly .2 points/month, you should consider increasing the KH or even performing partial water changes more frequently. KH does not affect fish directly, so there is no need to match fish species to a particular KH.
Note: it is not a good idea to use distilled water in your aquarium. By definition, distilled water has essentially no KH. That means that adding even a little bit of acid will change the pH significantly (stressing fish). Because of its instability, distilled (or any essentially pure water) is never used directly. Tap water or other salts must first be added to it in order to increase its GH and KH.
General Hardness (GH) refers to the dissolved concentrations of magnesium and calcium. When fish are said to prefer “soft” or “hard” water, it is GH (not KH) that is being referred to.
GH, KH and pH form the “Bermuda Triangle” of water chemistry. Although the three properties are distinct, they interact with each other, making it difficult to adjust one without impacting the other. That is one reason why beginning aquarists are advised NOT to tamper with these parameters unless absolutely necessary. For example, “hard” water often comes from limestone aquifers. Limestone contains calcium carbonate, which when dissolved in water increases both the GH (from calcium) and KH (from carbonate) components. Increasing the KH component also usually increases pH as well. Using the “sponge” idea, the KH acts as a sponge absorbing the acid present in the water, with the net result being an increase in the water’s pH.
Water hardness adheres to the following guidelines:
Note: dH means “degree hardness” and ppm means “parts per million” (roughly equivalent to mg/L in water). 1 unit dH = 17.8 ppm CaCO3. Most test kits give the hardness in units of CaCO3; this means the hardness is equivalent to that much CaCO3 in water but does not mean it actually came from CaCO3.
Salinity refers to the total amount of dissolved substances. Salinity measurements count both GH and KH components as well as other substances such as sodium. Knowing water’s salinity becomes important in saltwater aquariums. In freshwater aquariums, knowing pH, GH and KH is all that’s needed.
Salinity is usually expressed in terms of its specific gravity, the ratio of a solution’s weight to the weight of an equal volume of distilled water. Because water expands when heated (changing its density), a common reference temperature of 39F degrees is used. Salinity is measured with a hydrometer, which is calibrated for use at a specific temperature (e.g., 75F degrees is common).
One component of salinity that neither GH or KH includes is sodium. Some freshwater fish tolerate (or even prefer) a small amount of salt (it stimulates slime coat growth) and parasites (e.g., ick) do not tolerate salt at all. Thus, salt in concentrations of (up to) 1 tablespoon per 5 gallons can actually help prevent and cure ick and other parasitic infections!
On the other hand, some species of fish do not tolerate ANY salt well. Scaleless fish (in general) and some Corydoras catfish are far more sensitive to salt than most freshwater fish.
Caution: Add salt only if you are certain that all of your aquarium’s fish prefer it or can at least tolerate it.
The CleverArium Aquatic Care System can help you automate the various parameters of water chemistry for your aquarium, including scheduled, automated water exchanges. For more information: