One of the most important factors in maintaining an aquarium in proper condition is to ensure the biological equilibrium in it. This requires maintaining several important parameters such as pH-acidity, dH-hardness, oxygen, nitrogen and other elements. The chemical composition of water must vary depending on the type of fish and plants that are kept in the aquarium. Each species has its own individual requirements that must be taken into account, especially when creating a multi-species aquarium.

Water acidity (pH value)

This is a measure of the presence of acids in the water. A neutral pH is obtained when the pH of the water is 7. A reading below neutral is acidic and above that is alkaline. PH of water in an aquarium should be close to what fish are used to in nature. For example, the water in the Amazon River is acidic (about 5.5), while in some African lakes it is alkaline (about 8.5). In the first few weeks after the installation of a new aquarium, this should be done almost every day. This is done with special testers available in most zoological stores.

If the pH is too high, we can reduce it:

• With special preparations available on the aquarium market;
• by adding tannins: ready-made tannin or placing alder leaves, oak bark, oak leaves, fern roots or green walnut shells in the aquarium. Unfortunately, this is not always an effective way, it is quite time consuming, and in addition, some species of fish do not like a high concentration of tannins in the water;
• Adding orthophosphoric acid (a few ml of acid per 1 liter of water) or nitric acid (1 ml of acid per 1 liter of water). However, this method is not recommended for beginners and inexperienced aquarists.

If the pH value is too low, we can increase it:

• With special preparations available on the aquarium market;
• using sodium carbonate or sodium bicarbonate.

When adjusting the pH of the water in the aquarium, there are several important things to keep in mind:

• It is better to reduce or increase the pH slightly than to change it drastically at once;
• all changes should be made after carefully examining the reaction of the water and should be periodically monitored;
• do not allow too high a "shock" change in pH. It should be done gradually in several steps;
• pH changes are best done without the presence of fish and plants in the aquarium. Once the pH is adjusted, they can be placed in it.

Water hardness

This is the amount of salts dissolved in the water, especially magnesium and calcium salts. It is expressed in the so-called German degrees (º). 1n corresponds to 10 mg CaO dissolved in 1 liter of water. Aquariums are divided by water hardness into:

1. Very soft - 0 - 5º
2. Soft - 5.1 to 10º
3. Moderately hard - 10.1 to 20º
4. Hard - 20.1 to 30º
5. Very hard - above 30 degrees.

Keeping water at an appropriate hardness level is especially necessary when raising fish or plants in an aquarium. The water hardness can be measured with special water hardness testers available in some pet stores.

If the water hardness is too high, it can be reduced:

• by adding distilled water to the aquarium;
• by filtering the water through peat;
• using special preparations available in pet stores.

If the water hardness is too low, it can be increased:

• by adding harder water;
• filtering water through the calcium-containing material;
• using special preparations available in pet stores;
• by adding calcium or magnesium sulfate (not recommended for inexperienced aquarists).

Oxygen content

An important parameter of aquarium water is also the amount of oxygen dissolved in the water. The standard is considered the concentration of oxygen in water 5-7 mg / l at a temperature of 25° C. If this value is significantly higher, the fish in the aquarium may have problems with survival. If the oxygen deficiency is small, the fish breathe faster, but they generally feel fine. The situation will be much worse with a lack of oxygen in the water, as in this case the fish will simply suffocate. The concentration of oxygen can be measured by special testers or simply watch the behavior of fish (reddening of the gills, rapid breathing, floating on the water surface - all this points to a lack of oxygen in the water).

Nitrates and nitrites

These are toxic compounds, and their excess in the aquarium is dangerous. They are harmful to fish and can lead to their death. These harmful compounds come from decomposing organic residues, detritus and uneaten food. Nitrogen levels can be measured with special testers.

The main causes of their excess are:

• too many fish in the tank;
• dead fish and plants are not removed in time;
• too much food is given to the fish and they do not eat it all;
• poor filtration;
• infrequent preventive maintenance of the aquarium.

Other elements and chemical compounds in aquarium water

Aquarium water contains many elements and chemical compounds that perform their functions in maintaining the biological balance and are necessary for the development of plants and fish.

Iron is essential for good growth of aquarium plants. Its deficiency causes yellowing of leaves and poor growth. However, too much of it is harmful for fish and some of the more sensitive plants.

Carbon dioxide is needed for faster growth and reproduction. At CO2 concentrations of 10 to 40 mg/l the plants will show beautiful shades of green and fast growth. The CO2 concentration can be increased by using special CO2 equipment, which is becoming increasingly popular, especially in aquariums with many plants.

Copper is toxic and often leads to the death of aquarium fish. Its level can be measured with special testers. In case of high copper concentrations in the water, the entire water in the aquarium must be changed and prevented.

Chlorine, often present in tap water, damages the mucous membrane of the fish. Therefore, it is necessary to let the aquarium stand for a few days before releasing the fish to allow the chlorine to escape. The chlorine removal from the water can be accelerated by intensive aeration.

Phosphate is produced in excess when the fish are fed phosphate-rich feed or phosphate fertilizers in the water. An excess of phosphate leads to algae growth. This can be prevented by changing some water each week with fresh water, or by introducing fast-growing plants which use phosphate as fertilizer.