Aquarium salt: when to use it and how much per gallon

How much per tank

Count the water the tank really holds, not the label size: gravel and decor take some of it. Work out your water

Dissolve the salt in a cup of tank water first and pour it in slowly, away from the fish. For a water change, salt only the new water: a 5 gallon change takes 1 rounded tablespoon, whatever the tank’s size. Other brands of aquarium salt have their own labels; follow the one on your box.

When salt helps, and when it does not

Aquarium salt is sold to ease the stress of a new setup or a water change and to help fish recover from some illnesses. A tank with good water and healthy fish does not need it, and it does nothing for ammonia, nitrite or nitrate: a water change does. If fish are sick, find out what it is first; sick fish by symptom starts there, and a fish vet decides on treatment.

Aquarium salt is not marine salt for a saltwater tank, and it is not table salt with additives such as iodine or anti-caking agents.

Fish, shrimp and plants that do not take salt

• Shrimp and snails are the most sensitive; keep salt out of their tanks.
• Corydoras, plecos and other catfish, and loaches, are commonly kept without salt, as are other fish with very small or no scales.
• Live plants: API’s label warns that some are sensitive; soft-leaved and stem plants suffer first.

Fish from soft, acid water, such as most tetras and rasboras, have no use for it either. Livebearers such as mollies tolerate it best.

Salt stays until you change the water

Salt does not evaporate with the water and is not taken out by the filter, so the tank gets saltier each time you top up with salted water. Top up evaporation with plain dechlorinated water, and add salt only for the water you replace. To bring the salt down, change water without adding any; each change dilutes it by the share you replaced. Water-change amounts