FAQ: Water Care for fish

In an ordinary tank there is enough oxygen when the surface moves and the tank is not overstocked. Oxygen tablets or liquids are sometimes used in emergencies, such as transporting fish or a power cut, but they do not replace aeration and the filter.

At night plants and bacteria use oxygen, so levels are lowest in the morning, especially in densely planted or stocked tanks. Increase surface movement, run aeration at night and check ammonia, nitrite and the CO2 supply.

Yes, air fresheners, insecticides, cleaners or paint fumes can get into the water and be toxic to fish and shrimp. When spraying near the tank, cover it, switch off the air pump, which draws in room air, and ventilate the room.

Never on the inside, and it is best avoided outside too, since spray and fumes can get into the water. Clean the outside with a damp cloth or an aquarium-safe product, and the inside with a scraper or magnetic cleaner.

Yes, uneaten food rots, releases ammonia and phosphate, clouds the water and encourages algae and fungus. Feed small amounts the fish eat in a few minutes, and remove leftovers with a siphon or net.

Some rocks, sand or decorations can release minerals, lime or dyes, altering pH, hardness or even poisoning the water. Choose materials made for aquariums, rinse them well and test the water more often in the first weeks.

Usually the bag of fish is floated in the tank for a while to equalise temperature, then tank water is gradually added to it. For sensitive species slow drip acclimatisation is used. It is best not to pour the bag water into the tank.

The animals and their water go into a container, and tank water is dripped in slowly through thin tubing with a knot or valve until the volume has grown several times. Parameters change gradually, so this suits shrimp and sensitive fish.

Yes, in poor water fish get stressed, lose colour and are less active. Clean, stable water with the right parameters, together with a good diet and suitable surroundings, helps fish keep their bright colours.

The more fish, the more waste, ammonia and nitrate, and the less oxygen per fish. In an overstocked tank water quality deteriorates quickly, and even frequent changes and a strong filter do not always keep up, so stocking is worth planning ahead.

Aquarium salt is sometimes used short term, for example to reduce stress or nitrite's effect, or for some livebearers. Many freshwater fish, plants and shrimp do not need permanent salt and can be harmed by it, so only use it with a clear purpose and as directed.

Table salt often contains iodine and anti-caking agents that are undesirable for fish. If salt is needed, use aquarium salt or pure sodium chloride without additives, and for a marine tank only special sea salt.

Brackish water is the natural home of, for example, some puffers, archerfish, certain gobies and monos. Their water is made with sea salt, and salinity is checked with a hydrometer or refractometer according to the species' needs.

Marine water is made by dissolving special sea salt in reverse osmosis water as the manufacturer directs, mixing it with a pump for a few hours until fully dissolved. Check salinity with a refractometer or hydrometer and match the temperature to the tank.

A hydrometer is cheap and simple but less accurate and sensitive to temperature and bubbles. A refractometer measures by how light bends in a drop of water, is more accurate, but needs periodic calibration with a special solution.

No, only water evaporates and the salt stays, so evaporation is topped up with fresh water, usually reverse osmosis water. Topping up with salt water would steadily raise salinity. Salt water is only used for water changes.

It is soft, slightly acidic, tea-coloured water resembling Amazon or Southeast Asian forest streams. It is created with driftwood, dried leaves, alder cones or peat and suits tetras, bettas, apistogrammas and other soft-water fish.

Catappa leaves release tannins that slightly acidify and tint the water, so they are popular in betta, shrimp and soft-water fish tanks. As they break down they also become food for shrimp. Use leaves from a reliable source, free of pesticides.

Alder cones release tannins and humic substances that gently lower pH and tint the water. They are used in shrimp and soft-water tanks. Rinse them before use, add new ones a few at a time and watch the pH.

Liquid humic or peat extracts allow more precise dosing and quickly create similar water tint and properties without rotting leaves. They do not become food for shrimp or replace a natural setup, though, so they are often used together.