FAQ: Water Care for fish

During the day plants use up carbon dioxide and pH rises slightly, while at night CO2 builds up and pH drops. A small swing is normal, but in very soft water or with heavy CO2 injection it can be larger, so it is best to measure at the same time of day.

Used as directed and gradually, they are usually safe, but overdosing or changing pH too fast stresses fish. Check KH before adjusting pH, since it determines how strongly and for how long the product works.

Yes, driftwood releases tannins and humic acids that lower pH slightly and tint the water amber, especially in soft water. In hard water the effect is small, and soaking the wood before adding it reduces the tinting.

Calcareous rocks such as limestone, marble or coral pieces slowly dissolve and raise hardness and pH. That suits cichlids but not soft-water fish. You can check whether a rock is calcareous with a drop of vinegar: if it fizzes, it is.

KH shows how much carbonate and bicarbonate the water contains; they act as a buffer that prevents sudden pH changes. The higher the KH, the more stable the pH, while very low KH means pH can fall quickly.

GH shows how much dissolved calcium and magnesium the water contains. These minerals matter for fish health, shrimp moulting and plants. Different fish naturally live in water of different hardness, so GH is best matched to the species you keep.

GH measures calcium and magnesium, the general hardness, while KH measures the carbonates that stabilise pH. They often move together but not always: remineralised water, for instance, can have fairly high GH and very low KH.

Usually tap water is mixed with reverse osmosis or distilled water until the desired hardness is reached. Peat also lowers hardness slightly, while water from household softeners is unsuitable because calcium is replaced with sodium.

Hardness is raised with mineral products (remineralisers), crushed coral, calcareous rocks or special salts, for example for cichlids. Choose the product according to what you want to raise – GH, KH or both – and change it gradually, checking with tests.

In many parts of Lithuania tap water is moderately hard or hard because it comes from groundwater, but it varies a lot between areas. It is best to test your own water or ask the local water supplier.

It is water filtered through a membrane that removes almost all dissolved minerals and contaminants. It is used for soft-water fish, shrimp and marine tanks, but pure RO water has no minerals, so it must be remineralised or mixed.

Pure RO water has no minerals or buffer, so its pH is very unstable and fish and plants miss essential substances. Before use it has to be remineralised with special salts or mixed with tap water to the desired hardness.

It is a blend of mineral salts that puts calcium, magnesium and sometimes carbonates back into reverse osmosis or distilled water. There are versions for shrimp, plants or particular fish, dosed to the target GH or TDS as the manufacturer directs.

Distilled water, like RO water, has no minerals, so it can be used for topping up evaporated water or mixed with tap water. You must not fill a tank with it alone without remineralising. Choose plain water, not water for steam irons with added fragrance.

Mineral water varies widely and can be high in salts or carbonated, so it is usually unsuitable. Still, low-mineral water is sometimes used for small tanks, but check its pH and hardness first.

Only water evaporates and the minerals stay behind, so topping up repeatedly with tap water gradually raises hardness and dissolved solids. That is why regular water changes are needed as well as top-ups, and some people top up evaporation with RO water.

Neocaridina shrimp adapt to a wider range and often do well in moderately hard tap water, while Caridina shrimp usually need soft, more acidic water, often RO with remineraliser. Stable parameters matter most.

Malawi and Tanganyika cichlids live in hard, alkaline water in the wild, so a higher pH and hardness are usually kept for them. Crushed coral, calcareous rocks or special salts are used, and Lithuania's hard tap water is often a good base.

Dissolved carbon dioxide forms a weak acid, so the more CO2, the lower the pH. In planted tanks CO2 is added for growth, but too much can suffocate fish, so levels are monitored via pH and KH or with a CO2 drop checker.

It is a small vessel with indicator fluid that changes colour with the CO2 level in the water: blue means too little, green about right, yellow too much. It reacts with a lag of a few hours, so it is suited to watching the general trend.