PH – An introduction…

Del Clarke – Delmarine Aquatics

This post is an introduction to PH in aquaria. It is not a scientific study or a resource for your PHd (See what i did there?!)

It is my understanding of the importance of PH and a basic help for those particularly new to the subject. I am a hobbyist, no more, and have been so since 1983. I have never received any training in the study of water or its’ chemistry but have picked up a thing or two along the way i thought might help someone else.

If you read this post and just once say to yourself “Oh i didn’t know that?” then it has served its purpose. So lets get to it!

What is PH?

PH is the measurement of Acidity/Alkalinity, in this case of water.

It is measured on the PH scale from 0-14, with 7 being neutral.

From neutral, as the water becomes more acidic, the PH measurement drops. Each full number of measurement dropped indicates a measurement 10x more acidic than the previous number. I.e. a PH of 5 is 10x more Acidic than a PH of 6, which in turn is 10x more acidic than a PH of 7.

Conversely as the numbers increase, as does the Alkalinity. A PH of 9 for instance, is 10x more alkali than a PH of 8.

The image shows how this looks.

How do I measure it?

There are a variety of means available to you. You might take a sample to your Local Fish Shop (LFS) and ask them to test it for you (often they are happy for a small charge)
You would be better however if you take the whole fish-keeping thing serious to invest in your own means of doing so. This way you can test when you need to and not when it is convenient to get to an open and willing LFS. The most accurate/economical way means tends to be the liquid test kits.

These come either on their own as a PH Test kit or in a pack with other test options such as Ammonia, Nitrite, Nitrate, KH, GH, etc etc. (Lets not get into those now though they are all very important!) A good kit with all of the above would be an excellent investment.

There are also “PH Pens” which are digital measuring devices. You put the probes in the water and get a digital readout of the PH. There are many cheap options for these available online, but a word of warning. They need calibrating frequently and may be very unreliable. Do not trust them 100%. I am sure there are many versions far better than others out there but some may be beyond the average aquarist’s budget.

Why is PH important?

The water is the substance in which our tank life lives. Think how well we would do in our own environment if air was to be removed! The water, just like our air, has to be “right” to sustain life. Our air has to have sufficient oxygen (as does water!) and be free from harmful contaminants. One of the elements of the water that has to be right is PH. Plants and aquatic life can only thrive within a range of PH measurements. Typically, this is between about 5.5 and 8.5 though some can tolerate outside these measurements. However, any particular species has its’ own range within which it will flourish. South American Cichlids for instance typically live in water with a PH between 5.5 – 7.5. Obviously as in all things there are exceptions, but these are typical values. For many African Cichlids we might consider “normal” to be 7.5-8.5.
Does this mean an African cichlid will not survive in water of say, 7.0? No it doesn’t, it means if this is outside of its natural range it may not be the optimum for best of health/breeding. In some cases however the artificial environment may be so far from the natural range of a creature that indeed it may not survive to maturity.

Plants obtain many of their needs from the substrate and water around them. They too then can be similarly affected by the PH of their environment. For terrestrial gardeners out there, you may be well aware some plants like acid conditions (Low PH) such as Azaleas and Rhododendrons, whereas other do far better in Alkali (higher PH) soils (Hydrangea, Juniper, Lilac etc)

In the aquarium the PH, when outside the acceptable range, can cause the water to burn skin, damage eyes, prevent normal function of bodily organs and systems, including growth, reproduction, digestion, fighting disease, bacteria and parasites, affect moulting of crustaceans etc. so we really need to at least try to get it right. Your pets cant complain to you to let you know when it isnt!

How do I know what is “right”?

There is a wealth of information on the internet but perhaps the most relevant and useful information is from experience aquarists. Join a forum specific to your intended aquarium population (e.g. “Aquarium Shrimp Keeping” on Facebook) and ask the questions! There is no substitute for experience and there are some amazingly well-informed professionals and amateurs alike out there willing to help others.

What affects PH?
Pollution can affect rainfall (i.e. “Acid Rain”). As the water falls through the sky it can pick up pollutants that change the PH value of the water. This is a well-documented phenomenon with regard to Acid Rain and the effect is has on trees and plants etc. Pollution will also affect water seeping through contaminated soils, and when pollution finds its’ way into water courses it can affect the PH of rivers and streams.
Geology affects rain after it has reached the ground. As it seeps through the soils and rocks on and under the ground it can be affected by the very nature of these elements. Consider South-Eastern UK where underground is largely made of Chalk (Calcium Carbonate). This makes the water Alkali and therefore it has a higher PH than in some other parts of UK (>8.0) Other Areas may have a lot of peat in the soil which lowers the PH making it more Acidic.
Items added to Aquaria such as leaf litter and some decorative woods release tannins which not only turn the colour of the water yellow/brown but lower the PH making the water more Acidic. Some decorative rocks may release alkali into the water (e.g. Limestone) raising the PH.

How do I change the PH?
• First of all consider if this is necessary. If starting out with aquaria why not select tank inhabitants that suit the water you already have? This is by far the safest, cheapest and low-maintenance way of ensuring a happy aquarium colony and will save you tons of grief time and money!

Consider this: You go to your local fish shop and buy your fish/shrimp etc. They will most likely keep them in tap water. You bring them home to your own tank with water of a far different chemistry (beacuse you changed it!). In go your new acquisitions and what happens? PH Shock, stress, death. Changing your water parameters may be great for creatures that are accustomed to those parameters. Rapid changes in PH will kill far more fish and shrimp than having water a little outside their “normal” range. So again ask yourself, can I just have inhabitants that fit what I already have? If this is not an option, then there are ways to produce the desired PH.

 Make it! If you were to use a water filter system such as a Reverse Osmosis (plus optional De-ionising Stage/s) you can first remove much of the elements that affect PH to result in water close to 7.0 (Neutral) These “RO/DI” water filters don’t break the bank but will need to have some stages replaced over time as they lose their effect. I have a 4-stage RODI system that cost around £60 in 2019. This is then plumbed into the water system and produces clean neutral water for my tanks. I can then add preparations to affect the PH as I wish.

This is not the place to go into the different options for doing this, but remember to chat with those on the forums and find out what others do and what works best for them! Examples include Blackwater extract (to lower PH) or baking soda to raise it (yes, baking soda!)

Remember that water changes and time can affect the chemistry so water added may need to be made the same as is already in the tank to prevent the PH changing. Please do NOT dive into this without some considerable research and discussion/advice/guidance from others.

Tap water may be changed in the same way as above without the RODI system. However I choose to use it as shrimp are sensitive to other contaminants that may be present in tap water (such as copper etc). Fish and plants tend to be more tolerant of these.

PH Buffers – I use a buffering substrate in my tanks that lowers the PH to around 6.5 and holds it there. I do small water changes at a time (approx. 10-15%) so the neutral water going in doesn’t have a great effect, allowing time for the buffering substrate to “do its’ thing” and stabilise it at 6.5 again. These “active substrates” don’t last forever however and you should factor in how you will change/refresh them when the need arises. There are many available and again the forums will help you if you wish to go down this route.

The subject of substrates is a minefield and definitely not one to go into here!

PH Change.

Stability is paramount in aquaria. Rapid large changes of PH will stress and perhaps kill fish and other inhabitants so if you are looking to make changes, please remember to make them slow, gradual and measured.


This has been intended to be nothing more than an introduction to what PH is and the implications of it.

Having a basic understanding of PH, the needs of your tank inhabitants and how to provide them is a tool that will help you in your aquarium keeping. It is not the magic bullet to understanding everything your tank needs but is just one of many such things you will benefit from knowing.

Talk to experienced aquarists, professionals and amateurs alike, research the internet, your local library etc, to get a greater understanding of what is going on in that glass cube on its’ stand in the corner of your room.

I hope to write more of these posts and I am sure there will be many times I will be challenged on what I say. That’s great! This is how we learn and educate ourselves and each other. If you have any comments please feel free!