The FCA Guide: Aquarium Water Chemistry

african cichlid american cichlid fish fish care fish health pH water chemistry

When you think about setting up a new aquarium, what do you think about? - Fish, equipment, size? You cannot be more wrong - you should be thinking about the most important factor in fish keeping - the aquarium water!

If the water chemistry inside the aquarium water is not balanced, any living thing you put inside will die. Something a lot of fish hobbyists have unfortunately experienced. 

Some fish are pretty hardy, which makes them great for beginners. However, most are not, and any fluctuations in water parameters can be fatal. 

To prevent these problems, regular water testing from test kits are highly recommended, or you take samples to your local fish shop. 


Let’s look at the most common issue hobbyists can struggle with! 

pH is the measurement of acidity and alkalinity of water with 7 being neutral - below this is acidic, and above this being alkaline. If your fish requires a neutral pH, you are lucky as this is the easiest to maintain, however, this is not always the case!

Every fish has different requirements and ranges that they can tolerate, so you must know what your aquarium water should be. Most species of fish can adapt to a range of pH, however, this is only in very small changes. 

How can you change the pH?

This can be difficult, and if done incorrectly, can result in fish death. Whenever changing the pH, it should be done slowly and requires you to monitor levels closely. 

You can use substrates to naturally change the pH. For example, cichlids require a higher pH than other freshwater fish - so you can add limestone and coral-based rocks to create a more alkaline environment for them. 

Liquid pH additives are often used but are not recommended. - Aquarium water contains natural buffers to control the pH to the original value of when you added it into the aquarium. When adding the pH liquid, it chemically reacts with the aquarium water and neutralises it. When the buffer values decrease the liquid will have an immediate effect, resulting in strong fluctuations - becoming toxic to your fish. This is why they are not recommended as they are only short-term solutions. 

So, what is the best way to bring down the pH level in your aquarium?

The best way is to place wood and a commercial plant substrate in your aquarium or add peat to your filter. 

Commercial buffers also will decrease the pH levels, but they will need to be added every time you do a water change to ensure the pH of the water is the same as in the aquarium. 

Ok, so how do you raise the pH?

Crushed coral is a great natural chemical buffer! You can also use limestone chips in your filter which also increases the hardness.


Temperature is important to maintain when keeping fish as they are cold-blooded animals, so they cannot control their temperature. They are not able to add or remove layers of clothing as we do - so you need to control this, especially when performing those important water changes! 

When the water becomes too cold, your fish will become less active and have a reduced appetite - compromising their immune system and making them more likely to become sick.

Alternatively, if the water is too warm, their metabolism increases causing them to breathe more - consuming more oxygen. This is a serious problem, as warmer water contains less oxygen which can stress them out, cause organ damage, and in the worst-case suffocate them. 

Generally most fish require a temperature of 25 - 27℃, however, some prefer it a little warmer, or a little colder - be sure to research the specific requirements for the fish species you are keeping. 

To keep on top of this, a thermometer is essential. Stick-on thermometers are cheap and easy to attach to the tank. 

Some fish will require a heater, especially if you live in a country that experiences colder temperatures. Heaters come as submersible and hang-on heaters. 

If you live in a hot climate, you get the opposite problem - your water temperature will increase. You will want to increase the water movement as warmer water contains less oxygen. If you do live in a tropical climate you can buy a chiller, that can be placed internally (drop-in model), or externally, depending on what type you want. 

Ammonia & Nitrites

Ammonia is toxic to fish and corals. Ammonia comes from the waste and excretion your fish produce, uneaten food, and decaying plants in your aquarium. 

Ammonia is naturally removed by beneficial bacteria from the filter and substrate in your aquarium. When the beneficial bacteria consume ammonia, they convert it into nitrites, and then immediately into nitrates - which generally are not harmful to your fish. 

“My ammonia and nitrite levels are out of control!” - If this happens, it is usually from removing all the beneficial bacteria from your filter, performing a large water change, overstocking your aquarium, and/or overfeeding. 

What can you do?

In an emergency, you can perform frequent, small water changes. Add a dechlorinated solution to your aquarium, which should neutralise the water. Remove any uneaten food and decaying plants/material. - It is recommended that if your levels are super high, remove your fish into a separate tank until the water parameters are stable. 


Nitrates are not to be confused with nitrites. Nitrates are not a ‘red alert’ situation. But, over time they can accumulate and become a problem - generally, levels over 50 ppm should be a cause for concern. 

How can you keep the levels close to 0 ppm?

Densely planting your aquarium will help. Plants are great, they crave nitrogen, potassium, phosphorus - all of which are found in your fish’s poop, basically acting as a fertiliser for the plants! By adding plants, not only will they help with nitrates, but also create an aesthetic mini-ecosystem, recycling microelements in the aquarium water. 

Water Hardness (KH &GH)

KH is the carbonate hardness and refers to how much carbonate the water contains and determines the buffering capacity. We can also use KH to determine how much CO2 is in an aquarium.  

Why do we need to know about KH?

We need to determine how much CO2 to dose to control the pH when the aquarium light is turned off. 

**A higher KH = improved buffering capacity - you want to aim for ~80 ppm. 

GH stands for the general hardness of the water. Ever wondered why tap water can taste different? - This is all down to the GH of the water. 

Fish have a preference when it comes to GH. Most species can tolerate different ranges of GH, as long as it is not too high or too low. If you are into breeding your fish, GH is very important. 

**The higher the GH = the higher the pH levels will be. 

Raising  KH

To stop the pH from dropping, you want a higher KH. To raise the KH, you can add sodium bicarbonate in small amounts - you will want to do this when performing water changes.

How much sodium bicarbonate should I dose?

You should add 1 ½ tsp per 25 gallons - this raises the KH by 2  degrees. 

If you are struggling with the KH levels you can add crushed coral directly into the filter. Be careful when doing this, as crushed coral is likely to raise the pH levels of the water. 

Adjusting GH

This is relatively easy. You can add limestone chips into a filter bag - this increases the GH slowly. Coral can also be added to the filter or directly in the aquarium.  

Lowering the GH is not so simple and can be expensive. You will want to use reverse osmosis (RO) to dilute the water as this balances the amount of dissolved solids. RO contains no dissolved particles making it a great buffer and reducing the hardness, thus decreasing the GH.

A cheaper option is to add a bag of peat or wood into the aquarium to decrease the GH. However, this is very likely to also decrease the pH. 

Finishing Notes

I cannot stress enough how important it is to control your aquarium water. You need to ensure they are within the range of your fish, corals, and plants. 

Monitoring fish health is a good way to tell if something is not quite right inside your aquarium. 

Continuous testing and maintaining will prevent water parameter fluctuations and ultimately save your fish. Water testing kits can be bought online or at your local fish shop. 

Don’t forget to join our “Help & Advice” support group on Facebook. We offer FREE support from a qualified marine biologist!

Thank you for reading. We hope to see you soon, have a ‘fintastic’ day!

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