FAQ: Water Care for fish

Hot tap water can carry more dissolved metals from pipes and the boiler, so many fishkeepers use cold water and warm it, or mix in only a little warm water. Either way, use a conditioner and check the temperature.

Household softeners swap calcium and magnesium for sodium, so the water is not truly soft for fish and may not suit them. For an aquarium it is better to use unsoftened water, or reverse osmosis water if soft water is needed.

Well water can be high in nitrate, iron, heavy metals or other substances, and its make-up varies. Before using it for an aquarium, test the main parameters, especially nitrate, pH and hardness, and watch how the fish respond.

Rainwater is very soft but can carry pollutants from roofs, gutters and the air, especially in towns. If you use it, collect it after prolonged rain in clean containers, usually mix it with tap water and test pH and hardness regularly.

Fish waste and leftover food turn into ammonia, which some bacteria convert into nitrite and others into nitrate. Ammonia and nitrite are toxic to fish, nitrate much less so, and its excess is removed by water changes and plants.

It is the period in a new tank when bacteria that break down ammonia and nitrite settle in the filter and on surfaces. Until that happens these substances can spike, so many fish cannot be added at once.

A new tank's biological balance usually forms in 3–6 weeks, but it depends on temperature, the ammonia source and whether bacteria starters are used. Cycling is complete when ammonia and nitrite consistently read zero and nitrate appears.

Tests should show ammonia and nitrite dropping to zero within a day even after an ammonia source is added, while nitrate rises. Confirm this several times over a few days, and only then start adding fish gradually.

It is a way of cycling a tank where, instead of fish, the bacteria are fed by adding an ammonia source to the water, such as fish food or a special product. This spares fish from high ammonia and nitrite levels.

It can, but then add only a few hardy fish, feed very lightly, test ammonia and nitrite often and change part of the water when they rise. Bacteria starters and media from a mature filter shorten this period.

It is when too many fish are added to a new, uncycled tank and they get sick or die from ammonia and nitrite. You avoid it by letting the tank mature and adding fish gradually while testing the water.

After cycling, it is best to add fish in groups every week or two so the bacterial colony can adjust to the rising ammonia load. After each group, test ammonia and nitrite for a few days.

Yes, growing plants take up ammonia and nitrate, so water quality fluctuates less in a densely planted tank. They do not replace the biological filter, though, so ammonia and nitrite still need testing.

The bacteria that break down nitrite grow more slowly than the ammonia-processing ones, so the nitrite peak often lasts longest. Do not overfeed, keep the water warm and oxygenated, and if there are fish in the tank, change part of the water.

Cycling can slow or stall if the water is too cold, the pH very low, oxygen short, ammonia too high or too low, or the filter was rinsed in tap water. Check these conditions and, if needed, add a bacteria starter.

Yes, a handful of substrate, a rock or ornament from a healthy, mature tank brings bacteria and speeds up cycling of the new one. It can also bring diseases or snails, so only take it from a tank whose fish are healthy.

Ammonia damages fish gills and skin, makes breathing harder and weakens immunity, even in small amounts. Its toxicity rises with pH and temperature, so in a healthy tank it should be zero or close to it.

The most common causes are overfeeding, overstocking, a new or over-cleaned filter, a dead fish or snail, rotting plants and a filter that was switched off. Ammonia can also come from tap water treated with chloramine.

Do a partial water change with treated water at the same temperature, feed sparingly for a few days, remove anything rotting and check the filter. Some conditioners bind ammonia temporarily, and repeat the tests daily until it drops to zero.

Fish may gasp at the surface, breathe rapidly, be sluggish, clamp their fins and have reddened gills. If you see these signs, test the water at once, change part of it and, if things do not improve, contact a vet.