The best water for aquariums is well water: it’s clean, clear, odorless, and free from any decaying substances. You can also use water from pristine streams or springs, as long as it doesn’t have any unpleasant smells or tastes from certain mineral salts and their impurities. The same goes for well water. Well water is suitable if it is clear, odorless, and has no unpleasant taste.
If your aquarium houses only one species of local fish, it’s often best to use the water they were previously living in, if possible. Clean rainwater can also be used for the aquarium, provided it was collected directly in clean containers and not runoff from dirty, painted roofs.
In cities where tap water is the easiest option, you can fill your aquarium with it only after letting it sit in a separate clean open container for at least 24 hours, allowing the chlorine to evaporate.
Here are some additional tips related to specific situations and phenomena in the lives of fish and plants in the aquarium.
The appearance of foam on the surface of the aquarium indicates that the water is low in oxygen, especially when fish rise to the surface and gulp for air. Sometimes foam can also appear when fish have overeaten. In both cases, you should add a few cups of cold fresh water to the aquarium or aerate the water.
If you notice a mucous-like film on the water’s surface, it suggests that dead animals or uneaten food are decaying in the aquarium. As soon as these clouds or films appear, you need to change all the water in the aquarium, rinse the plants, and add new substrate; otherwise, the fish may die in the decaying water. Before doing this, you can try to skim the film off the surface and add a bit of salt or saltwater to the aquarium. If the film reappears, then proceed with the described disinfection.
A grayish film on the water resembles the previous one but is completely harmless. You can collect this film by gently submerging a cup into the water. The film will flow into the cup along with the water. It’s even easier to use a bottle, submerging it so that the opening is almost touching the water’s surface—the film will be drawn in by the water flow entering the bottle.
A whitish film on plants, resembling mold, sometimes covers bread, cheese, etc. This is a colony of harmless microscopic organisms known as “infusoria,” which can be gently wiped away with a piece of cotton.
Sometimes tiny worms, thin as threads, appear on the bottom of the aquarium. These worms are harmless and serve as good food for fish.
Cracks in the aquarium glass are most often caused by improper placement, such as tilting or bending of the frame. To prevent the glass from cracking, it should be placed on a flat surface. If the glass does crack, it should be immediately sealed with wax. If water starts leaking from the crack, you must first drain all the water from the aquarium, align the glass pieces together, and seal the crack with hot wax.
Cracks in aquarium walls can also be sealed with a special putty. The best putty for inserting glass into an aquarium is a mixture of Portland cement and oil varnish. Pour some cement into a metal or clay container, add a bit of oil varnish, and mix until a thick paste forms. Before inserting the glass, coat the grooves with this mixture. The putty should dry well for three days before you can fill the aquarium with water.