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Basic Water Chemistry

PostPosted: Sat Nov 29, 2008 6:35 pm
by Crazygar
Basic Water Chemistry
How it relates to Planted Tanks

Water Chemistry

The new hobbyist and even some long timers will often be concerned about the hardness and pH of their water as it relates to keeping fish and plants healthy and thriving. Understanding hardness, alkalinity, and pH is important to having a firm grasp on tank health and to aid in diagnosis when things go wrong...or more importantly, to alert you before things can go terribly wrong. Let's slowly take the three main parts and break them down into simple terms and discuss the why's, how's, and effects that each may or may not have on keeping your fish and plants healthy and thriving. Water chemistry can be made to sound complex and for a very thorough understanding of what happens to metals, phosphate, and buffers it is very complex...however, it need not be made into a complex issue for even the advanced aquatic gardener or fishkeeper. A basic understanding of terms and how they need to apply is all we need for successfully maintaining a healthy thriving aquarium filled with tropical aquarium plants.

Water Hardness (GH)

Water Hardness as it's related to fishkeeping is measured with a General Hardness (GH) test kit and it is measuring the amount of calcium and magnesium combined in your water. The more calcium and magnesium present in your water the "harder" your water. New people to plants will often have read misleading information regarding growing plants in soft or hard water. Many authors have erroneously concluded that soft water is better for plants and that hard water makes keeping plants healthy more difficult. Over the past couple of years it has become increasingly obvious that the reality is just the opposite. Since calcium and magnesium are both macronutrients it seems logical that having those nutrients wouldn't be problematic. Using CO2 injection is more critical for hard water tanks though and may be why hard water is seen by some authors as a determent. According to Claus Christensen of Tropica "those plants that are typically considered "soft water" plants are simply those that do a reasonable job of surviving and growing in very soft water. These same plants, in rich hard water, grow MUCH better." This reflects my experiences having tried both soft and hard water, hard water produces better results.

There are circumstances where the fish you want to keep may need soft water to show their best color, remain healthy long-term, or maybe for breeding purposes. Unfortunately it is very easy to increase water hardness, however lowering water hardness is more difficult, or at least lots more costly in terms or either time and/or money. In order to make hard water soft you have only a few viable options.

(a) You can dilute water hardness by using a mix of RO (Reverse Osmosis) water and tap water. Or, you can use all RO water that has been treated with a commercial product such as Kent's RO Right to restore enough hardness and buffer to the RO water so that it can support life. Straight RO water is "dead" water and essentially won't support life.

(b) You can collect rain water and filter it with Activated Carbon (AC) for a couple of days prior to use. Use the filtered rain water to dilute your hard water by mixing a percentage of rain water and tap water.

(c) Filtering with peat, or making a peat "tea concentrate". Not the best choice as the results are not very predictable and dependable. The pH of the beginning peat and the ratio of peat to water makes a big difference on the hardness and pH lowering capabilities of peat. Using peat while also using CO2 injection is not recommended because the acids from the peat will interfere with the results of your KH test and skew the CO2 charts so that determining actual CO2 levels becomes very difficult.

(d) Ion exchange. Depending on how this is done is could be an alternative or it might be a disaster in waiting. Whole house water softeners will soften water by exchanging calcium and magnesium for sodium. For your fish nothing much has changed, in fact for some soft water fish these extremely high levels of sodium are quite problematic. Ditto for plants...this was a bad exchange for them, removing two macronutrients for something they don't really need. You could mix some of the softened water with unsoftened water and lower your GH some but again, you are adding unwanted sodium so why go there, choose the RO route instead. Some water softeners can use potassium chloride instead of sodium chloride and those units are much less problematic. The only real issue to be concerned with is making sure that adequate amounts of calcium and magnesium are still present. Water that has high amounts of potassium can block the uptake of calcium unless calcium is present in a fairly large quantity too.

To summarize, don't ever soften water for your's not necessary. In hard water CO2 injection is more important and you will spend less money and a lot less time by employing a pressurized CO2 injection system rather than an RO unit.

Carbonate Hardness (KH)

This is not a measurement of hardness. It is a measurement of Alkalinity. I wish they would call the test a Total Alkalinity test. Alkalinity is the amount of buffer in your water, or rather the ability of your water to resist a change in pH. Alkalinity is what neutralizes acids and keeps the pH stable. Components of Total Alkalinity are the bases bicarbonate, and carbonate, and occasionally hydroxide, ammonium, borates, phosphates, silicates, sulfides, and organic ligands. But in freshwater and at the normal pH's we are dealing with the Alkalinity is almost exclusively bicarbonate. In addition to keeping the pH stable, Alkalinity is also important in regards to metal concentrations as bicarbonate and carbonate in water can remove metals by precipitating the metals out of solution.

While some plants can get carbon from stripping bicarbonate, it happens at an energy cost to the plant. Rather than forcing plants to acquire carbon this way it is best to provide ample CO2 so that they can meet their carbon needs the easiest way possible. In order to safely increase CO2 levels in an aquarium we need to be able to measure those CO2 levels. Fortunately there is a very simple process for determining how much CO2 is dissolved in water and the Total Alkalinity measurement (KH) is critical to the process. When CO2 dissolves in water some of it (a know exact amount) forms carbonic acid. Carbonic acid behaves uniquely with bicarbonate and in simple terms, allows the pH to drop because of the carbonic acid but does not effect the KH...which is unlike all other acids. All other strong acids will cause a drop in KH with a corresponding drop in pH...though in most cases the buffer is strong enough to resist any changes if the acid addition is small. Back to CO2 and carbonic acid, the acid lowers the pH while the KH remains the same, it is this difference that allows us to measure how much CO2 is dissolved in water. For most water with a KH of over 1 degree the pH is set by the amount of bicarbonate or KH. The higher the KH, the higher the pH. CO2 dissolves into water readily and has an equilibrium rate of about 0.5 ppm with the atmosphere outside. However, in our tightly closed homes the CO2 levels indoors are generally 2-3 times higher. With normal fish respiration and biological processes in our tanks along with the atmosphere CO2 most tanks will reach equilibrium at 2-3 ppm of CO2. This is the rate that a tank without CO2 injection would remain at unless it had plants removing CO2 faster than it can dissolve at the water surface. Plants removing all the available CO2 is a situation that is very likely to occur with moderate to high lighting and/or heavily planted with fast growing plants. If plants run out of CO2 some species can use bicarbonate, unfortunately when CO2 is all used up the pH can rise very high and begin a process called biogenic want to avoid this nasty situation where the pH can rise to 9.0 or higher and cause calcium carbonate to precipitate. As already mentioned CO2 dissolves readily into water and we can inject CO2 to levels of 20-30 ppm safely without harming our fish and that also insures plants are never carbon limited.

What's important to remember is that increasing the amount of CO2 will lower pH while the KH remains the same. When CO2 levels drop the pH will rise but the KH will stay the same. In other words if you inject CO2 and lower your pH one full point from 7.8 to 6.8 your KH of 7 degrees would remain the same. If you were to stop injecting CO2 the excess CO2 above equilibrium would outgas and drop back down to reach equilibrium...and your pH would return to 7.8 while your KH would remain the same. To determine CO2 levels we need to have an accurate pH reading and an accurate KH reading, Those readings are compared to CO2 charts that factor in carbonic acid effects and allow you to see on a graph how much CO2 is dissolved in your tank. Here's a typical CO2 chart. ... tic&id=445

And here is an excellent downloadable CO2 calculator.

At this point it needs to be clearly understood that to measure CO2 requires that your buffer be almost entirely bicarbonate/carbonate and that no other strong acids or bases are present. Because the KH test kit measures Total Alkalinity it will "count" the acids such those from peat filtering or phosphate based pH controllers as part of the KH and give a false high reading. This false high reading will make the CO2 chart invalid showing significantly more CO2 in your tank than is actually the case...due to the skewed KH test kit results. The point here is this, don't change the buffer of your water by using peat or phosphate based pH controllers...leave the buffer to be a bicarbonate/carbonate one.


More people probably worry and fret over this number than anything else in fishkeeping. Beginners are always concerned their pH is too high or too low. Sadly many authors have given rise to this myth that fish need a specific pH to thrive. While it may be true that some fish need a low or high range pH to spawn and have the eggs hatch the truth is an overwhelming number of fish will live, breed, and eggs hatch and thrive in a very wide range of pH. Much more significant is TDS or Total Dissolve Salts...Total Hardness. Plants really don't seem to care about pH...but they do better with plenty of CO2 in the water and when that occurs the pH is going to fall near neutral. The important thing to remember about pH isn't that your fish require a specific pH but rather a range of GH and KH that will effect the pH, in that sense pH can seem to be important. I'm also unconcerned about pH swings if due to an influence in CO2 levels. And I firmly disbelieve that there is a "pH shock" that many authors refer to. I keep many "considered" sensitive fish such as Apisto's and Chocolate Gourami's. I've never experienced any sign of stress related to sudden and dramatic changes in pH in fish tanks. I routinely move fish from CO2 injected tanks to non-injected tanks...again no problems in either direction.

I've never seen any problems nor do I subscribe to the belief that there is such a thing as "pH shock". Wright Huntley has some interesting posts on the APD if you search the archives for "pH shock". Here's my take on the issue of pH shock. In and of itself pH swings cause no harm, but the impact of a movement in pH may have synergistic effects. Meaning that it's a combination of two or more substances which produce effects greater than their sum. For example, the pH of the water will determine the toxic effects, if any, of many substances. Ammonia, Nitrite, and metals have toxicity based on pH. Provided your water is pristine and devoid of ammonia and nitrite then pH swings should never cause any stress. Moving fish from water with low TDS to high TDS is often tolerated too, but moving a fish from high TDS to low TDS will often kill them in minutes. I never move fish from water with differing amounts of TDS...never.

What to do if my water lacks buffer (low KH)

If you chose to inject CO2 you really need to have a KH of at least 3 degrees. Starting with a lower amount of buffer means your pH is lower too. Since injecting CO2 to levels of 20-30 ppm requires a drop in pH of 0.6 to 1.0 it's safest to do so with a starting pH of at least 7.4 so that at 30 ppm your pH is no lower than 6.4. Experienced fishkeepers can operate safely at levels below this if they have special circumstances but for most the prudent thing to do is to begin CO2 injection with 3 degrees or more of KH. To increase KH you can use baking soda. Baking soda is sodium bicarbonate. Bicarbonate/baking soda will raise the KH of 50 liters by 1 degree per 1/4 teaspoon. Baking soda will only raise the KH, if your water is too soft you may wish to use calcium carbonate to raise both GH and KH. Calcium carbonate will raise both KH and GH of 50 liters of water by 1 degree per 1/2 teaspoon. Alternately you can place crushed coral, cuttlebone or dolomite in your filter to dissolve slowly and increase both KH and GH but understand that this method is inexact and inaccurate.