Modifying Water Chemistry
Aquarium Resources

Modifying Water Chemistry

In many cases, our fish desire water conditions where hardness and pH-value are not the same as in our tap water. It is therefore necessary to know how to modify water hardness and pH-value to make it more suitable for our beloved pets. In many situations, aquarium fish may very well survive in unadulterated tap water, but once you have learned how to provide them with ideal conditions they will thank you by looking more colorful and perky than ever – many species will even change into their vibrant breeding dresses and start to procreate.

Modifying water chemistry in a freshwater aquarium  

Increasing water hardness and pH-value

Baking soda, Epsom salts and Marine salt (or calcium chloride)
If you need to raise the water hardness and pH-value in your freshwater aquarium, e.g. to make it suitable for African Rift Valley cichlids, you can use equal parts of baking soda, Epsom salts and a marine salt mix (or calcium chloride).

You probably have to go no further than to your own kitchen to find some baking soda. The chemical formula for this everyday household product is NaHCO³ and it will add plenty of bicarbonate ions to the water of your aquarium. Adding 1 milliliter of baking soda per 10 liters of water will raise the alkalinity up to approximately 110 mg/L CaCO³. (10 ml/L roughly equals 1 tsp/10 gallons.) Bicarbonate ions can be used to increase the pH-value up to 8.2, but it will have no effect on the water hardness.

To increase the water hardness, you will instead need some Epsom salt and calcium chloride. Epsom salt is the market name for hydrated MgSO4 and adding 1 milliliter of it per 10 liters of water will increase the permanent hardness by roughly 70 mg/L CaCO3. You may however wish to increase the calcium (Ca++) contribution to hardness as well and this can be achieved by using a marine salt mix or calcium chloride. It may feel strange to add salt to a freshwater set up, but you only have to add a really small amount of salt to increase the calcium content to suitable levels for African Rift Valley cichlids and other calcium loving freshwater species. Such a small amount of salt will not harm fish species used to hard water. Pure calcium chloride will naturally also increase the calcium content, but calcium chloride is normally must more difficult to come by than a marine salt mix. Instead of picking it up in the pet store, you may have to order it directly from a chemical supply store.

Calcitic gravel
Using calcitic gravel will naturally also affect the water hardness in your aquarium, but only up to about pH 7.5. If you have acidic water and wants to increase the pH-value a bit, calcitic gravel is great, but if you have alkaline water that you need to make even more alkaline (e.g. to house the abovementioned African Rift Valley cichlids) you need to use other methods because calcium carbonate is so insoluble in alkaline water that it becomes virtually impossible for it to push the pH-value above 7.5 in the aquarium. Many aquarists still include calcitic gravel in their hard, alkaline aquariums because it will function as a form of insurance and may very well save the life of your fish is something goes bad and the pH-value starts to drop dramatically. With plenty of calctic gravel in the aquarium, you pH-value will be very resilient and rarely drop below pH 7.5.

Decreasing water hardiness and pH-value

There are a lot of fish species that appreciates soft and acidic conditions, such as the myriad of cichlids that inhabits the soft and acidic rivers and streams of the South American rainforest. Commonly kept and captive bred fish species have normally adapted to harder and less acidic conditions and may even breed in such conditions, but there is also a wide range of interesting fish species that have not. Learning how to make your water softer and more acidic is therefore a good idea.

Unstable water conditions
Before you start tinkering with the pH-value in your aquarium, I wish to add a word of caution. If you lower the pH-value without simultaneously buffering the water to the lower value, you will force the pH-value down under its natural level and this creates a very unstable situation in your aquarium that can shift rapidly when you turn your back. The same thing is true if your use CO2 injections to lower the pH-value – if the CO2 injection system stops working the pH-value will increase rapidly and such as drastic change can prove lethal to aquarium fish.

It should also be noted that fish produce acidic waste products and if you allow the amount of waste to build up in an aquarium where the pH-value is already low, it can drop down to unhealthy or even lethal levels. Monitor your pH-value closely and strive to keep a stable environment. If you are constantly combating dropping pH-values, you should reduce the fish load, increase filtration, clean the filters more often and/or increase the size and frequency of the water changes. You can also consider raising the alkalinity somewhat.

Lowering alkalinity
One of the easiest methods of achieving low alkalinity in the aquarium is to use naturally soft water to begin with. You can for instance collect rainwater, melt (clean) snow or purchase treated water. Water can for instance be treated through distillation or reverse osmosis (RO). You can then use harder water or baking soda to increase the alkalinity up to the ideal level for your fish species.

If you live in an area with a lot of pollution and/or little downpour, the precipitation may not be suitable for aquariums it can contain harmful compounds or be filled with too much dust. Getting your own RO unit is then an option. They aren’t cheap, but if your alternative is purchasing distilled water, they will prove financially sound in the long run.

Lowering the pH-value
Once you have obtained water of desirable alkalinity, you probably need to tinker with the pH-value as well. Increasing acidity will make the water well-buffered, thus increasing the stability. Using chemicals, e.g. hydrochloric acid, to decrease the pH-value is not recommended since it will cause rapid changes and create an unstable environment.

You can find pH-decreasing chemicals in most pet shops, but most of them will unfortunately consist of sodium biphosphate which serves as great nutrition for algae. Using this type of pH-decreasing chemical can therefore cause an algae boom in your aquarium. Acid buffers free of phosphate are a better choice.

If you want to stay away from the chemical bottles, you can use pure sphagnum peat moss to lower the pH-value in your aquarium. Peat moss releases organic tannins to the water and will therefore increase acidity and lower the pH-value. It is actually organic tannins the makes the blackwater rivers of South America so acidic. Some aquarists dislike the tea colored water, but if your fish hails from a blackwater environment it will appreciate it a lot. 

CO2 injections
Injecting carbon dioxide is yet another way of lowering the pH-value, but it can cause a lot of problems unless you know exactly what you are doing. I therefore recommend you to read a more in-depth article about CO2 and water chemistry before you go about. CO2 injections will lower the pH-value without simultaneously increasing acidity; hence produce an unstable situation in the aquarium. You will for instance have to be careful with water changes, since the new water will have a high pH-value compared to the aquarium water unless you inject CO2 into the replacement water as well. CO2 does however have its upsides; it will for instance promote lush plant growth (provided that your plants receive sufficient amounts of nutrients and strong light).

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