What water to add during water changes.
There has long been a debate about whether water for water changes needs to be aged or prepared in another way. There is no unambiguous answer to this question; it is best if you know how tap water is treated in your water supply system. Usually, if you are changing a small portion, you can pour water directly into the aquarium, while ensuring there is no large temperature drop and spraying the water to allow chlorine to escape and to oxygenate it. Sometimes water conditioners for chlorine removal are added to the aquarium at the same time.
You should add water whose temperature and hardness (and preferably pH level as well) are the same as those of the water in the aquarium before the change began. If you add any elements to the water (for example, to increase hardness), it is best to do this while aging the water or gradually, as it is added. This avoids concentration spikes. In some regions, especially during droughts, it is advisable to measure the nitrate level in the tap water every time before adding it to the aquarium, as a seasonal increase in this level may be observed under such conditions.
When tap water is mixed directly with aquarium water, gases dissolved in the tap water may be released after the pressure drops. This can be recognized by many small bubbles settling on the glass, internal decorations, and the bodies of the fish. They can cause gas bubble disease in fish! Before adding fresh water to the aquarium, it is necessary to bring its chemical composition and temperature into line with the chemical composition and temperature of the water in the aquarium. Such measures will help avoid the risk of phenomena such as pH shock, temperature shock, and gas embolism.
Aquarists generally use tap water. Any water treatment plant has an information department where you can get data about its hardness, pH level, nitrate content, and, finally, chlorine content. During primary treatment, chlorine and its bactericidal compounds are added to the water first of all. By the way, chlorine is not always present in tap water, because in some cases ozonation is used in the water supply system for water disinfection.
In high concentrations, chlorine is toxic. For most aquarium fish, the threshold chlorine concentration in water is 0.25 mg/L, and a concentration of 1 mg/L is lethal. At lower concentrations, it also harms fish by damaging the gills. Heavily chlorinated water (this is usually achieved with chlorine) can always be distinguished by its smell. According to accepted water treatment standards, elemental chlorine is no longer present in water at a pH of 6. In principle, the drinking water of our water supply network should not contain more than 0.3 mg/L of active chlorine.
The main methods for removing chlorine from replacement water are aging and chemical treatment. However, the danger of chlorinated drinking water is usually overestimated. Chlorine is relatively unstable in water, independently evaporating into the atmosphere. Chlorine escapes from water when it is in strong motion (fountaining, foaming). Therefore, it makes sense for an aquarist to keep new water prepared for a partial change in a special container for 1-2 days, although if you install an aeration device there, the aging time can be significantly reduced. The water aging tank should have a sufficiently large surface area for interaction with the air environment.
By the way, in an established aquarium, water changes can be made with unaged tap water (and not everyone has the opportunity to age water for a large aquarium), simply doing it more frequently and in small volumes. In most cases, replacing 20% of the water with tap water has no negative impact on the fish. Many aquarists pour chlorinated water straight from the tap into the aquarium. Indeed, this can be done because when poured into a well-established aquarium, chlorine is neutralized within a few seconds through interaction with the local environment and does not have time to cause harm.
Although, for example, for some species of Lake Tanganyika cichlids (*Cichlidae*), even these few seconds during a massive water change turn out to be enough to kill them. In this situation, what is fatal is not so much self-poisoning by chlorine as the stress from a sharp change in water parameters in the aquarium. Therefore, the amount of water replaced at one time must be coordinated with the degree of sensitivity of a particular fish species, and if the fish was previously kept in an aquarium where chlorine-free water was added, it should be acclimated to chlorine gradually, increasing the volume of water changes in stages.
However, it is still better to add water after pre-filtering it, especially since this is easy to do. It is particularly dangerous to use chlorinated water during the spring flood period, when the chlorine concentration is increased for better water disinfection. Chemical treatment. The fastest way to remove chlorine from water is to use a chemical dechlorinator preparation, which reacts chemically with the chlorine dissolved in the water, turning it into low-toxicity substances. Sodium thiosulfate $Na_2S_2O_3$ is usually used for this.
Sodium thiosulfate is contained in water preparation products popular among aquarists. But when using them, keep in mind that the dosages indicated in the instructions are averaged, while the chlorine content in tap water can vary depending on the locality, time of year, and other conditions. A more reliable method is the adsorption method—passing chlorinated tap water through activated carbon; chlorine is adsorbed on it, and the water is freed from the toxic component.
Another chemical substance used by water supply enterprises for water disinfection is chloramine, a mixture of chlorine and ammonia. It is believed that chloramine “has significant advantages because its bactericidal effect lasts longer than that of chlorine, and besides, chloramines do not impart an unpleasant odor and taste to the water, unlike chlorine and its compounds with substances contained in the water.” From the perspective of aquaristics, however, everything is completely the opposite.
Chloramine is a huge evil and a headache. Sodium thiosulfate can neutralize only a part of chloramine—the chlorine, while ammonia remains in the water, although there is an assumption that chloramine still does not completely dissociate in water and forms complexes that can be adsorbed by carbon. Fortunately, in our country, the practice of chloraminating tap water has not become widespread. However, it should be borne in mind that chloramine can also form independently as a result of the interaction of tap water chlorine and ammonia.
Why do the walls and plant leaves become covered with air bubbles after a water change in the aquarium?
One can often observe how many air bubbles settle on the walls of a vessel filled with white tap water that looks like milk. This is oxygen dissolved in it under pressure. If such water is not allowed to age and fish are placed in it, or a lot of such water is added to the aquarium, excess air enters the fish’s blood through the gills and, releasing into bubbles, can block small blood vessels.
Blockage of blood vessels by air bubbles—gas embolism—occurs in fish when using unaged tap water. In fish suffering from gas embolism, the gills darken, and blood edema appears on the body and fins. After normal gas exchange is established in the aquarium, these symptoms may disappear, and the fish recovers. With severe blockage of blood vessels, ulceration appears, and the fish may die.
How often should you perform water changes?
Large water change in the aquarium. Using a filter for water changes. 
