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Adjusting pH in the Freshwater Aquarium

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One of the most common problems encountered by the aquarist is maintaining a constant pH level. Many hobbyists fail to understand the importance pH plays in the aquarium and what factors influence the pH reading.

When considering pH, rather than having a goal of a specific reading, you should have a goal of stability. Although 7.0 may be the optimal pH level for a given fish, the same fish will likely thrive at a constant level anywhere between 6.6 and 7.4. It may even survive at constant levels between 6.2 and 7.8. It is the drastic swings which can occur in pH that we as hobbyists are trying to prevent. Our goal as aquarists should be to establish a very consistent pH level, even if that level is slightly outside the “ideal” pH reading for a given species. In other words, a constant pH of 6.6 is better than a pH value which fluctuates between 6.6 and 7.0, even for a fish which prefers a 7.0 reading.

The property of water to resist changes in pH is known as buffering capacity. You can determine the capacity of your buffering system by measuring total hardness. A reading of 4-6 dH or higher is usually adequate to keep the buffering system in place and maintain a stable pH. A reading under 4 dH means there isn't enough of a buffering system and the pH is likely to drop. For higher pH levels, you will probably want to aim for 6-12 dH. Many hobbyists choose to measure only Carbonate Hardness (KH), which is a measure of the calcium carbonates in your water. This test is also effective in maintaining a proper buffer system. When testing for Carbonate Hardness, a reading of 75-100 mg/L is adequate for most aquariums, while a reading of 100-200 mg/L would be desired for higher pH levels. For the purpose of freshwater aquariums, measuring either total hardness or carbonate hardness is necessary, but measuring both independently would not be needed.

Fortunately, your aquarium water usually has a natural supply of dissolved minerals which make up a buffer system to keep your pH stable. All of these dissolved minerals together make up the total hardness of your aquarium. As aquarists, in addition to testing the pH of your water, you need to test for total hardness. Knowing your pH reading alone does not tell the entire story. Only by testing total hardness can you predict that your pH will remain stable after you adjust it to the appropriate level. This is due to natural biological processes that occur in your aquarium, such as fish respiration and organic waste breakdown, which result in acids that neutralize the bicarbonate ions that make up the buffer system. Aquarists replenish these buffers and increase total hardness with partial water changes, and help keep the buffer system intact with routine aquarium maintenance, such as cleaning filter pads and vacuuming the substrate.

I would like to briefly address why each of these parts of your aquarium maintenance routine are necessary for stabilizing pH in the aquarium. You need to know that anywhere in your aquarium where detritus (a fancy term for dirt) accumulates is a source of Phosphate production. As detritus accumulates in your gravel bed and on your filter pads, the Phosphate levels in your aquarium rise. Free Phosphate ions may bond with calcareous buffering material, precipitating calcium from your aquarium, and reducing your aquariums ability to keep pH stable. This is why it is so very important to clean your filter pads regularly and vacuum the aquarium gravel with each water change. In addition, your tap water contains buffering ions. Doing regular partial water changes will help to replenish the buffers which have been lost. This is important in all aquariums, because fish respiration and organic wastes alone will cause a gradual drop in the ability of your aquarium to buffer against pH swings.

Now the question becomes what to do if the fish you want to keep have very special pH requirements. If your fish prefer a pH level which is reasonably close to the pH your aquarium water is naturally buffered to, then I do not recommend you make any changes at all. Unless you are keeping an extremely specialized fish your fish will be fine. On the other hand, if your fish have pH requirements which are far from the values in your tank, then you have work to do.

Let us consider methods of raising the pH of your tap water. There are many additives on the market today which claim to raise your pH. I have found most of the liquid products to be 50/50 success at best when used alone. You also need to use a product to increase the buffering ability of your aquarium. To maintain a stable pH in the upper levels of the pH scale for fishkeeping, I would recommend using a buffering substrate such as crushed coral. You can add crushed coral to your existing aquarium. I personally place the crushed coral in a mesh bag and place the bag under my gravel substrate. You will want about 1 kg of crushed coral per 40 liters of water to buffer the water to hold a pH around 7.6. Alternatively, some hobbyists choose to place a mesh bag of crushed coral in their power filter. This method does not allow for the use of large quantities of crushed coral, but can be effective if you only need to make small adjustments to your aquariums buffering ability. If you prefer not to used crushed coral, there are many buffering powders available on the market which are effective at increasing the hardness and/or alkalinity of your aquarium. You simply add the powder to your aquarium water according to the directions. This method tends to create more rapid changes in your water hardness, which can be dangerous if you are not experienced in adjusting hardness and pH. The safest method is to use crushed coral. By first increasing your aquariums ability to buffer the water, you can now adjust your tap water pH to the appropriate level and add it to your aquarium. There are many products on the market to accomplish this. It is often the case that your tap water naturally has a high pH and no adjustment is necessary. You probably only need to add crushed coral to help your aquariums buffer system maintain a high pH.

Lowering the pH is a more extensive task. If you simply add a product such as “pH Down” this will not work. Your buffering system will simply very quickly raise the pH back to its original state. You must remove the buffering ions from your tap water so that you may lower the pH. The best way to accomplish this is to purchase a Tap Water Purifier unit. These units filter the water from your faucet using an ion exchange resin. The resulting water is free of the salts and minerals which buffer your water. Aquarium Pharmaceuticals makes a nice compact Tap Water Purifier specifically designed for aquarium use. This is the only reliable method I know of to reduce the buffering ability of your aquarium water and to lower pH. Without purified water, you may be able to lower your pH for a day or two, but without first removing the buffering ions your pH will climb again to natural levels. This fluctuation in pH is much worse than having the wrong pH to begin with. As an additional option, I should mention that many hobbyists use peat to soften their water. By running peat in your power filter, or by placing a layer of peat under your gravel, you will soften your water. This technique can work well, but is more complicated, less predictable, and probably best avoided by the inexperienced hobbyist.

I have personally found that it is easier to buy fish which fit the pH of your water supply, especially if your tap water has a high pH. There are literally hundreds of fish available in the hobby which can thrive in the 6.6 to 7.8 pH range. I am certain that regardless of your pH level, there are fish which you will enjoy keeping and which will thrive in your aquarium.

Be assured that attempting to control pH is the most frustrating experience for a new hobbyist. I would guess that 50% of the problems encountered in new aquariums are a result of the aquarist attempting to change the pH level. Few fish keepers actually need to adjust their pH. For the majority of aquarists your tap water pH will be adequate. The dangers of adjusting the pH incorrectly far outweigh any benefit you may receive by moving your pH a few points on the scale.

Remember, when it comes to adjusting your pH, less is more! Stability is most important. Routine maintenance is the key to keeping your pH stable and your fish healthy!

Happy fishkeeping,

Reader Comments Comentário

In the text it is said that a build up of phosphates will end up destroying the buffering capacity of the water, but my understanding has always been that phosphates are themselves buffering ions, so I believe it would be more accurate to state that an accumulation of phosphates will tend to buffer the water at a lower level. The difference is that a phosphate-buffered water in the wrong pH should be almost as hard to correct as a carbonate buffered one, in contrast to the idea that a water that lost its buffering capacity is easy to correct.

My most painful personal aquarium disaster occurred several years ago when I decided to use a phosphate remover in a 4 year old planted tank with some of the favorite fish I've every kept, just to help me with algae control. It turned out that over the years the water had been buffered at about 6.5 by the phosphate and when the resin removed it the pH swung to 8.5 or more overnight, killing nearly all of my 4 year old fish :-(

Finally, the Tap Water Purifier method certainly works very well as described, but I lived for 2 years in a city where the tap water was extremely hard and alkaline, and as a consequence the resin lasted very little and it became very troublesome and costly to replace it all the time. The solution I found at the time was to simply go to the supermarket next door and stock up gallons of distilled water for the water changes. At a cost of US$0.29 per gallon it was a much cheaper solution (at least for my small tank), and all I had to do was mix in a bit of tap water before using it in the tank.

Contributed by Marcos Avila

I am so amazed to see that someone has addressed the issue of phosphates in a freshwater tank! Most don't address the issue unless you are specifically looking for a reason for an algae bloom or are into saltwater tanks. Being new to this hobby I need as much info to maintain a healthy aquarium. I have purchased and read several books and have done tons of research on the net. Most books and websites simply stress the need for basic tests, i.e. ammonia, nitrites, nitrates, but not phosphates. I had no idea I should be testing for phosphates, until my tank crashed. One major algae bloom and nine dead fish later, I have learned a lesson at the expense of my poor fish. My phosphates were so high that within seconds of adding the last chemical reagent to the test tube, the results surpassed the highest on the color key. After numerous water changes and adding phosphate pads to both my filters, my phosphates are now within acceptable ranges and we are now adding a few new fish. Thank you so much for this important information, I didn't find it until it was to late, but now I know and this will NOT happen to my fish again!

Contributed by Cathi Rey

Two years ago I used crushed coral, placing it in with my gravel; it slowly raised and kept the pH constant at around 7.2; unfortunately, over time, the coral broke down and caused the water to become extremely cloudy. The only solution in the end after trying all types of filtration etc. was to empty my 270 liter tank and remove all coral, a most tedious and exhausting task. Now I'm waiting for my tank to recycle. I, therefore, do not recommend using coral unless you are willing to pay a high price in the end.

Contributed by

I live in an area where the tapwater isn't very conducive to fish health, with an alkalinity hovering around 8.5 and hardness maxing out commercial test strips. My solution was simple and affordable. I filled my tank with distilled (reverse osmosis) water from the local supermarket, and then added freshwater aquarium salt to give myself the correct ion composition for buffering. I'd say the total cost of setting up the water in a 280 L tank was under $20. I will do small water top-ups with treated tapwater, but larger water changes are done with distilled to keep the hardness withing the range of test strips. As a side note, a pleco is an excelent indicator of water hardness, at least my bristlenose is. When the water gets too hard he simply stops moving and lies on his belly on the bottom of the tank/rock (not suctioned upside down or against anything) and will actually refuse to move unless poked. I do an agressive water change and he's back to chasing the loach.


I'm happy to find a discussion about pH and phosphates. I have well water that contains a high carbonate hardness level, that keeps the buffer for pH very high (80 ppm). The only solution I have found is to bring in water from the city that uses reverse osmosis in their filtering (0-4 ppm). I assumed that testing for general hardness was enough. Carbonate hardness is the most important in managing the pH of an aquarium. Only after speaking with the water plant engineer and running tests in the lab that I work in, did I figure out how to get around the buffer.

Contributed by Angela Denes

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