Salt is one of the most valuable treatments you will ever use in a freshwater aquarium. It is a great way to naturally and safely treat many types of illness and general stress.
Salt treatment is also very effective against ich and other external parasites. A full dose is one tablespoon per five gallons. It is best to start with a half dose and then put in the other half dose after about twelve hours. This is especially true if you have any fish that may be sensitive to salt such as loaches, tetras, discus, etc. In most cases there will be no issues but every once in a while a fish will have a bad reaction to the salt. In these cases the may become lethargic, breathe rapidly, or have clamped fins. In these cases you will want to do a water change to remove some of the salt. Goldfish and livebearers (mollies, platies, swordtails, and guppies) react very well to salt treatments. Keep in mind that you need to dose for salt again after each water change. So if you have a full dose of salt and then do a 25% water change then you need to add a full dose of salt for the 25% you changed. So if you have a 20 gallon and do a five gallon water change you need to add a full dose of salt for those five gallons, one tablespoon.
The usual dose is one tablespoon per five gallons. It is best to start with a half dose and then add the other half 12 hours later in case any fish have an unusual negative response to the salt. This response would be general signs of stress (clamped fins, lethargy, reduced appetite, increased breathing rate, etc.).
Salt use in freshwater aquariums is a highly debated and misunderstood concept. There are definitely some very good and important uses for salt in a freshwater aquarium.
Salt can be one of the most powerful and effective treatments for stress and many illnesses. It is something that should be on hand for any aquarist. When something goes wrong in an aquarium and the fish are stressed for some reason the first thing that should be done is a water change. If that is not enough the next thing that should be done is to add some salt. The standard dosing is up to one tablespoon per five gallons. I would not add all of this at one time. I would add about half and wait about twelve hours before adding the other half. This will allow the fish to acclimate to the sudden increase in salinity, which can be stressful to some species. It is also good to keep an eye on the fish after each addition of salt to make sure that none of them are responding poorly to it.
Some fish respond very well to salt treatment. These fish include goldfish, livebearers (guppies, mollies, swordtails, and platies), and others. It is amazing how so often all that is needed to treat what seem like severe problems is water changes and some aquarium salt. This is a much more natural and less stressful way of handling many issues than harmful, toxic chemical medications.
The problem with salt is that it is often suggested to be added all the time, with every water change. These are freshwater fish, not brackish or marine. They do not need salt added to their water. They have adapted to freshwater chemistry, that is what their kidneys can handle. Adding salt all the time, especially at the dosing suggested by the companies selling salt who want you to buy a lot of it, is not good for the long term health of the fish. Lake Tanganyika is one of the hardest, most alkaline bodies of freshwater on the planet. Seachem’s Cichlid Lake Salt has a recommended dose of 1/2 tablespoon per ten gallons. The usual recommendation for aquarium salt to be added to freshwater aquariums all the time is one tablespoon per five gallons. That is four times as high as one of the hardest and most extreme bodies of freshwater on the planet. Subjecting all freshwater fish to this salinity, especially soft water fish, is going to cause harm long term. They simply are not adapted to handling such a salinity, their physiology is not able to handle it safely long term.
It is important to address other salts in freshwater aquariums. For example, Seachem’s Cichlid Lake Salt. Many people feel that since most African cichlids are captive bred they have adapted to a wide range of chemistry. This is true to a point, but these fish evolved in in very hard, alkaline waters with a unique mineral content. Special salts made specifically to mimic this natural chemistry may not be needed for these fish to do well, but in my experience the fish do even better when their water is closer to what is found in their natural habitat. Their colors will be a little better, their health will be a little better, they will thrive better long term. Something as simple as a special salt additive to make such a difference for them is worth it, they deserve it.
Livebearers and Salt:
Livebearers requiring or doing better with salt is another highly debated topic.In nature many livebearers go in to brackish and even fully marine water. They do this in search of food, but they do not live there permanently. Their bodies have adapted to handle this short term exposure to high salinities. In captivity they do not have the option to go back in to full freshwater if the aquarist adds salt all the time. They are forced to have to deal with these extremes in salinity all the time. This can cause more harm than good long term. In addition, even if livebearers did do better with salt all the time, it doesn’t matter if there are any other freshwater fish in the tank since they definitely cannot handle the higher salinity all the time.