Aquatic Life Farm, LLC

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Keeping Fish Tips

How to Control the pH for Your Fish

by Frank Cowherd

It sounds like it would be a no-brainer, but have you ever tried to maintain a low pH because you want to breed an acid loving fish or a high pH because you want your Rift Lake cichlids to look great? Depending on what fish you want to get to spawn or maintain in a healthy condition, you might need to provide water at any pH between 5.5 and 8.5.

pH    1  2  3  4  5  6  7  8  9 10 11 12
 |  |   |
very acidic
very basic 
Range for fish is approximately 5.5 to 8.5

Click on highlighted text to go to section for producing specified pH

Because most aquarium fish are adaptable, they can usually survive in water outside of the pH range of the water they are found in naturally. In fact most tropical freshwater fish can live in regular tap water, minus the chlorine or chloramine, because it comes out of the faucet at between pH 7.2 and 8.5. This pH range is the result of two things.

First the source of the water (rivers and lakes or deep wells) has a fair amount of minerals that are dissolved by the water to produce a pH in this range.

Second, the water company treats that water by filtering it through sand and/or by adding clarifying agents to remove debris and then, if the pH is not high enough, they run it through crushed carbonate rock or add lime to purposefully raise the pH into this range. They do this to prevent corrosion of any metal pipes carrying the water to your faucet. Metal is corroded less by water that is basic than by water that is acidic.

If a water company supplies your water, its pH is usually going to be above neutral. If not, call the water company and find out why it is acidic.

There are exceptions to tap water having a pH above 7. If you have a shallow well, the water in the well may not come into contact with carbonate rock so its pH is more likely to be affected by organic matter through which it passed. When organic matter like leaves or grass decays, it produces acids and removes hardness, making the water more acidic and softer.

For most cases you will not have to do anything with your tap water except remove the chlorine or chloramines for aquarium use. But,there can be two problems, both of which are fixed by aeration. It is good to aerate any source of water for a while to make sure it contains oxygen at the right level. On the one hand, the tap water can contain too much dissolved gases, which is caused by the high pressure needed to move the water from storage to your tap. This excess gas is bad for fish. Bubbles appearing on the sides of your aquarium and other objects are evidence this condition. On the other hand, the tap water could contain very little oxygen or could contain harmful gases. All of these possibilities are remedied by aeration.

So, with a little dechlorination and a little aeration you will have good water with pH between 7.0 and 8.5 that is suitable for keeping and even spawning a lot of different tropical freshwater fishes. Let’s call this water normal tap water.

Normal tap water will slowly shift to a lower pH as the process of living (by the fish and the plants) goes on. Fish produce wastes, and decay of this and other organic matter, such as food and plant debris, produce acids. So the pH of a tank filled with normal tap water will not stay where it started without your doing something, as described below.

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Maintaining pH Near or Above 8

African cichlids from the Rift Lakes need basic (versus acidic) water that is maintained at or above pH 8 to be healthy and reproduce, although some will argue that a pH around 7.8 is fine. There are a number of ways to provide this pH starting with normal tap water.

The easiest is to provide a carbonate-containing substrate. Such materials are readily available at your local fish store (LFS). Examples are crushed limestone, crushed coral, and crushed oyster shell. The carbonate-containing material reacts with the acids produced from living and decay, thus removing these acids and maintaining the pH.

You can also maintain the pH by monitoring and adding either sodium bicarbonate, that is sold as baking soda in grocery stores, or by adding hydrated lime which is also called calcium hydroxide, slaked lime, slack lime or pickling lime and is sold in garden centers in 5-pound bags. Bicarbonate will get the water to about 7.8, while hydrated lime will easily get the pH above 8.

You can do all three things together: use the substrate, add bicarbonate and add hydrated lime, or you can do any one of them. The substrate is easiest because it will continuously neutralize the acids formed and you do not have to monitor the pH frequently. With the other two you will need to monitor the pH frequently until you learn how often and how much to add.

And do not worry about maintaining a precise pH like pH 8.0. The fish can easily handle some swing in the pH.

If you decide to use hydrated lime, the following note might be of interest. Hydrated lime is usually sold as a fine white powder, which is not very soluble in water. So when you sprinkle it into a tank of water, it coats everything with a fine white powder. The dissolved portion of the hydrated lime consumes any acids and keeps the pH above 8. The portion that did not dissolve originally acts as a reservoir to replenish the dissolved portion as it is consumed. Thus, it acts much as the carbonate-containing substrate with the advantage that you can tell when it has been used up. Also, it is not that critical how much hydrated lime you add although moderation is best. To avoid the unsightliness of the white powder over everything, it can be added to a filter or sump so you do not have to put it in the main tank. Periodic additions of hydrated lime are needed to maintain the pH above 8.

For Rift Lake cichlids or any other fish that need high pH water and like harder water, you will probably want to add some Rift Lake salts that is sold in your local fish store (LFS) or some Epson salts (magnesium sulfate) to make the water a bit harder. If you are adding hydrated lime, it is calcium hydroxide, so you are already adding calcium ions. Hard water almost always has a high pH and vise versa. But note that the addition of sodium chloride (table salt) cannot be used to increase the hardness. Only calcium and magnesium salts are counted as hardness.

If you Google “synodontis petrocola”, you can find a description for breeding them at a pH up to 9. To achieve this pH hydrated lime is used. The other two additives will not produce this high a pH.

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Maintaining pH Near Neutral

Most tropical freshwater fish, particularly the ones found in community aquariums, live well and reproduce well in water that is in the 6.8 to 7.5 pH range. It is possible to provide water in this pH range just using normal tap water. It might start out at a pH near 8, but the processes of living and decay will bring the pH down. Without water changes the tank can continue to drift down to a pH as low as 6, given sufficient time. Most fish tolerate this slow drift even as low as to pH 6 easily because it is gradual, over months. But they cannot stand a rapid change in pH.

The typical aquarium is set up with normal tap water and contains a normally high load of fish and plants. The fish are fed once or twice a day. If the pH is measured once a week, it will be found that the pH will be down at close to a pH of 7 within 3 weeks. The exception is for water that is very hard, in which case the pH will take longer to get to neutral.
Once reaching a pH near neutral, that pH can be maintained by doing partial water changes on the order of 10 to 30 percent weekly or biweekly or monthly. Start with a ten percent water change and see what the resulting pH is. Then adjust the percentage water change accordingly.

The lowering of the pH caused by the acids produced from the process of living/decay goes on in all aquariums and in nature. The acids are associated with a light-yellow color. So, in fact, you can tell if a tank does not get regular water changes by how yellow the water appears. You can demonstrate this yourself if you put some of the tank water in a clear glass on a piece of white paper, and compare it to a glass of water from the tap. Very yellow water is a bad sign because it indicates that not only is the pH low but the water has a relatively high salt content from adding water to make up for evaporation. For a tank is this condition, change only a small amount of water, but do it frequently (daily, for instance) until the yellow is gone or almost so. Changing a large amount of water on such a tank can lead to loss of fish and plants due to large changes in either or both osmotic pressure and pH.

One other note: be sure not to use any carbonate-containing substrate or decoration, like seashells, in a tank you are trying to maintain at pH between 6.8 and 7.5. This material can keep the pH above the desired range.

If you have very hard water, it may not be possible for you to use the above procedure to maintain a pH near neutral. Hard water and high pH provide a very large amount of acid consuming material. The softer the water is the easier it is to use the above procedures to achieve a near neutral pH.

However the above procedures can be used with rainwater or distilled water or RO water with good results. None of these sources of water contain any buffers, so they will not change the pH. They are usually slightly acidic at about pH 6.5 to 7.0. Using them to replace water during a water change will help maintain the pH of the water before the water change. In fact a good way to maintain a neutral pH is to mix normal tap water and one of these three sources of essentially neutral pH water. If your tap water is very high in pH, only a very small of portion of tap water is used with a large amount of the other.

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Maintaining pH Below 6.8

There are some fish that survive but maybe do not thrive at a pH near neutral or above. So you can keep them alive in a community aquarium for a relatively long time, but for them to thrive and spawn you have to provide a low pH. Fish that require such low pH come from forested or vegetated areas whose waters are soft and acidic because of the decaying vegetation through which the water flows. Mostly these low-pH waters, like the Amazon River basin, do not contain any carbonate rocks. If it did, the rocks would have dissolved in the acidic water raising the pH and adding minerals.

Examples of fish requiring low pH to reproduce are some pencil fish (pH 6.5), most peat-diving killifish (pH 6 to 7) and cardinal tetras (pH 5.5). On the other hand, rams, angelfish, and many tetras come also from such acidic waters, but, as most of us know, they will survive quite well in water at a higher pH. Angelfish and rams will even spawn successfully at pHs greater than 7.5. To breed discus and tetras it is better to lower the pH to less than 7.

To provide water with a fairly stable pH below 7 you need to consider a number of things.
First, is your normal tap water useable?
Second, how much do you want to spend on the project?
And third, how low a pH do you need?

If you have hard water, it will be almost impossible for you to bring it down even near neutral and still do routine water changes while maintaining an acid pH. There is a possibility of using it if you are willing to use a lot of chemicals. But then you have low pH water with a lot of dissolved chemicals in it, and the fish are not likely to like it.

Even if you have only moderately hard water, it will take additives to get the pH below neutral. The fish may be fine with the mineral level, but they might not spawn and you will not know this for some time. There are better ways to get a low pH.

One option that I have used in a pinch is to buy distilled water at a dollar a gallon. Distilled water is soft and has a pH of 6.5 as measured in all of the bottles I have purchased But again you cannot do routine water changes with your typical tap water and still maintain an acid pH unless you use additives.

There are only three practical routes to generating water with an acidic pH.
• The cheapest route is to use rainwater. But this makes you dependent on the rain. If you collect it from the roof, the first 15 minutes of rain should be discarded, as it will contain all the stuff that settled on the roof since the last rain. Collect it in a barrel, and put some fish in it to eat the mosquitoes.
• Another relative cheap way to generate acidic water is to use peat moss.
• The expensive, but easiest way, is to buy a reverse osmosis/ion exchange (RO/IE) unit.
These will be described below.

None of the chemical methods for generating acidic water are practical. There are acids like hydrochloric acid, phosphoric acid (found in the colas you drink), and acetic acid (vinegar) than could be added to lower the pH to whatever pH is desired, but this pH value cannot be maintained for more than a few hours, and the concentrated acids are harmful and difficult to handle. Then, there are acidic buffers; mainly mixed salts of organic acids but these are not very useful in the aquarium because of the large amount of these buffers that would be required, particularly if starting with normal tap water.

Peat Moss
Peat moss not only lowers the pH but it also absorbs minerals. So you can start with normal tap water. You can either set up a large aquarium filter packed with peat moss or soak peat moss in a barrel of water. As the peat moss contacts the water, it removes some of the salts making the water softer and adds organic acids that lower the pH. If the initial amount of peat moss does not get the water down to the pH you want, change out the old for a new batch of peat. Peat moss produces water that the fish will do well in.

You can even boil the peat and add the cooled, dark extract to the water to achieve a low pH. These two peat possibilities are good low-cost ways to achieve a low pH whose value is maintained by turning on and off the peat-filled filter or adding more black water extract from the boiled peat. Routine water changes using water treated with peat should be done to maintain the pH and the water quality.

Perhaps the lowest cost way is to place a partial bale of peat in a barrel of water (rain water or normal tap water) and wait until the pH is below the values you want. Then transfer the water to a tank and carefully dilute it with tap water to the pH you want. But take care, as only a very little pH 8 tap water will raise the pH of the treated water higher than you want. Start with a few ounces of normal tap water to 10 or 15 gallons of treated water.

One note must be made about peat moss. Peat mosses are not all equal. Depending on where (and how?) it is harvested, it can have very different abilities to remove ions from water and lower the pH. The best way to get a good peat is to get one that has a low pH. I understand that the best peat you can find is around pH 4. The manufacturer who bales the peat should know what the pH value of his peat product is. If you cannot get this information, you just have to buy different peat moss products and test them by putting some in water and measuring the pH versus time.

Reverse Osmosis/Ion Exchange
The easiest, but not the cheapest, way to achieve a low pH is to use water from a reverse osmosis (RO) unit that is coupled with a mixed ion exchange (IE) unit. Such units can be purchased new for around $250 and will last for years. This combination will produce water that has virtually no dissolved salts or ions (very soft) and a pH of around 6.5. This pH value is stable over a week or so, particularly if you are feeding live foods and do not allow debris to accumulate in the tank. My experience with maintaining pH 6.5 is with doing a weekly fifty-percent water changes using fresh RO/IE water. With that process and feeding live foods, I have been able to maintain pH 6.5 for months and have successful spawnings of coral red pencil fish. I do however add a pinch (1/16 teaspoonful) of marine salt to forty-five gallons of RO/IE water because the fish do better with at least a trace of salts in the water. Water at this pH is good for breeding not only pencil fish but also most of the difficult killifish, discus, and most tetras.

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Producing pH 5.5 Water

The peat-filled filter is used to lower the pH of the RO/IE water from 6.5 to 5.5 in order to get fish like neon tetras to breed. (See previous subheading for information on peat and RO/IE.) The time it takes to get the pH to this lower level depends on the amount and pH of the peat moss used. Using black water extract from boiling the peat will also bring the pH down. If you start with the soft water from a RO/IE unit (pH 6.5) or rainwater, peat treatment to produce water at pH 5.5 is not difficult. Starting with normal tap water and peat will take longer.

The difficult part may be finding a pH test that will give good readings in the 5 to 7 range. Many readily available pH test kits for the aquarium hobby go as low as pH 6.

When you are maintaining the pH of acidic aquarium, you must take considerable care not to add any untreated tap water, particularly if it is hard and basic. Only a few ounces of hard and basic tap water will raise the pH of an acidic aquarium significantly. Using normal tap water to feed a slurry of microworms, baby brine shrimp or other foods to your fish in the acidic tank will result in a pH increase with just a few feedings. Rinse your brine shrimp and any other live foods in water with the right pH prior to feeding them to the fish in an acidic aquarium.


In summary you now know what it takes to provide water at various pH levels. If you are one of the lucky ones, you have a faucet that supplies water at a pH of around 8 and a RO/IE unit that produces water at pH 6.5 and some low-pH peat moss and some baking soda and some hydrated lime. These few things can produce almost any water condition required for breeding any particular fish. OK, so maybe you need to know what is a good carbonate-containing substrate and what is a good non-carbonate-containing substrate (sand, quartz gravel), and what constitutes sufficient aeration to remove bad gasses and add oxygen to the water at the right level before you put fish in it. But overall it is not too complicated to provide your fish with water at a pH that should make them comfortable and maybe even help them become romantic.

April 22, 2009

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