First Tank Guide Tank Logo
First Tank Guide Home

Aquarium pH

What About Adjusting the pH in My Aquarium?

(The First Tank Guide)

The most common questions about aquarium water chemistry are about pH. In almost all cases, these questions and their related concerns are unnecessary. Most fish will thrive in a wide range of pH, and different fish have different ideal pH requirements. Though some exotic fish are more particular about the pH they require, even most of these fish are only particular about pH when they are breeding - and the only bad side effect of not maintain an ideal pH or making the right pH change at the exact right time is that the fish will not spawn. Also, most of these fish, if for no other reason than the demands on the pH are not good beginner fish. Unfortunately, there is a growing trend in the pet industry to believe that the pH in a fish tank needs to be 7.0 or very close to that. In fact, nothing could be further from the truth.

Questions about pH

Most of the pH related questions I get are from someone who has used a chemical to adjust their pH and has either had no effect or has had disastrous side effects.

Both of these effects are common and expected, and, usually, they are due to the same cause.

What is pH?

pH is a measure of the acidity or alkalinity of a substance. pH is measured on a scale of 1-14 with 7 being neutral. Something with a pH lower than 7 is acidic. Something with a pH higher than 7 is basic. Water has a pH of 7 naturally, but the water you are using in your tank will be different because of the chemicals that are suspended or dissolved in the water. These chemicals fall into three categories: acids, bases, and buffers. Acids are chemicals that lower the pH, or make the water more acidic. Bases are chemicals that raise the pH of the water, or make it more basic (or alkaline). Buffers are chemicals that can 'tie up' acids or bases and keep the water at a specific pH. Different buffers will keep the pH at different values.

What if I Do Want to Change the pH in My Aquarium?

If you decide you want to change the pH in your aquarium, and you are lucky, then your water will not contain any buffers. This makes changing the pH very easy. If you are lowering your pH, you add an acid to the water and as this acid neutralizes all the bases in the water, the water becomes neutral. As you add more acid to the water, the pH continues to drop and the water begins becoming acidic. You can continue this until the pH is where you want it.

However, if your water contains buffers, then things become more difficult. Again, if you are lowering the pH in the tank, you add an acid which needs to neutralize the bases that are in the water as above. However, even after the bases are neutralized, the water will maintain a high pH because of the buffers in the water. You will need to add enough of your acid to neutralize the buffer. However, usually, when you are overcoming a buffer in the water, by the time you have added enough acid to overcome the buffer, you will have enough acid in the water to cause a precipitous drop in pH.

Though almost all fish are very tolerant of a wide range of pH in the water they live and thrive in, sudden and/or drastic changes in pH, as those caused by overcoming a buffer in the water, are almost always harmful. Not only to your fish, but also to your plants and to your biological filter.

Of course, in most cases buffers in the water are good for your tank for this very reason. If the pH is buffered to a specific value, then things that are added to the tank will have a harder time changing the pH, so your water will remain more healthy and stable.

Now, remember, if you are changing the pH of the water, you will need to do this every time that you do a water change, and slight variations in the treatment of processing of the water you are using can make dramatic changes in the behavior of these buffers. Also, objects in the tank can also make a big difference in how the water is buffered. This means that you cannot, reliably, adjust the pH of your water before you put it in the tank, and this, in turn, means that if you are messing with the pH in your tank that you will have to subject your fish to this pH roller coaster every week when you do your weekly water change.

And all this for a pH change that probably isn't necessary!

Why Isn't it Necessary to Change pH?

Remember, most fish will thrive in a wide pH range - usually from as low as a full point below their ideal pH to a full point above the ideal. Also, different fish have different ideal pH as a base for these ranges. Some fish prefer pH as low as 5.5 and others prefer their pH to be over 8.5! Remember, before you consider messing around with the pH of the water in your aquarium, there are several questions you need to have the answers to:

  1. What is the ideal pH for each of the types of fish I am keeping?
  2. What is the actual pH of the water I am keeping these fish in? Not just is it high or low, but what is the value on the pH scale from 1 to 14. Just knowing that the pH is above or below 7 really doesn't tell you very much.
  3. Is the actual pH within the range for these fish? (Chances are that unless the water causes burns when you wash with it, that it is tolerable for almost any fish you could get.)
  4. Is there actually a reason to adjust the pH, or am I just doing it because it seems like something to do?
  5. Am I seeing any problems with my fish? Have I ensured that I am providing the necessary care for the fish including weekly 10-15% water changes, regular necessary filter maintenance, proper diet and feeding, correct temperature, correct salinity, and healthy tank population? Chances are, if you are seeing problems, that one of these tasks is not being completed, and it is the cause of the problem, and adjusting the pH will make things worse rather than better.

Quite possibly, the worst thing you could do to your fish tank is mess with the pH with chemicals. The pH is going to be buffered naturally to whatever it is coming out of the tap, and is very difficult to adjust safely. Adding chemicals to the water will very often leave you with an unstable system, constantly fluctuating and ready to plummet or skyrocket as soon as you put anything else (your hand, a fish bag from the pet store, a decoration or some fish food...) in the water. I would strongly discourage anyone from trying to mess with the pH of their aquarium water unless they really need to and they really understand chemical titrations.

Again, most fish can tolerate a wide range of pH, different fish preferring different pH, but thriving in a wide range and tolerating a wider range.

I strongly suggest that only experts with a good background in chemistry mess with pH, as any little mistake is asking for disaster. I have only been in this hobby since 1980 and would not consider myself sufficiently an expert to mess with pH using chemicals...

What Can I Do to Change the pH Slowly and Safely?

If you want to lower your pH safely, add a piece of wood to the tank as a decoration or add some peat to the filter system, but be aware that this will stain the water yellow or brown for a couple of years. If you want to increase pH, add a sea shell or a coral skeleton. Remember, these will not give you immediate, overnight results, but they will introduce buffering agents to the water to help keep the pH more like you want it.

"You have a very nice aquarium web site going on [here]. It is always nice to see a site set up with the health of the fish in mind. Not just what kind of fish are out there but the proper set up and care is included in your site. I understand how much time it takes to set on up and maintain it. They kind of grow on you like the hobby itself."
April 19, 2003
More Comments
"Your site is wonderful. Of all the sites I have consulted, yours is the one with the most information and very well explained."
January 16, 2007
More Comments