@Gemma, there is alot of info on this forum about salt and good aquaculture articles about this as well on the Google... but also... You have to realize that "pond salts" are made to be sold, to make money for the business... which is not the same as saying they are necessary for your situation...
Short answer...
"Sodium" type salts (like table salt) are only used if you will be significantly transporting the fish, maintaining incredibly high fish densities in your pond (most often done by hobby fish breeders or farms), or are treating for bacterial infections in a quarantine tank. "Sodium" type salts also can be used as a parasticide, but this procedure is more complicated, should
never be done in a pond, and likely never necessary for hobbyists to do.
Only use the calcium chloride salt whenever you want to increase your GH (which should be at very least 120ppm, and up to 250ppm is quite fine) or when you notice Nitrite numbers suddenly climbing (which only then a one time dosage is all this is needed for this).
Long Answer...
I figure you are talking about "sodium chloride" salt. Interestingly enough is that fish's blood actually has a salinity of (0.8%) and most of this salinity is composed of sodium and chloride while the remainder is bicarbonate, potassium, and calcium.
Sodium and potassium salts are critical for heart, nerve, muscle, and Sodium Potassium pump functions. The Sodium Potassium pump uses the Sodium to help the fish flush out excess ammonia from their body. One function of the mucus surrounding fish's body is to slow down the loss, due to osmosis, of these salts while the fish is in the water, but the mucus does not stop the loss.
Much of these lost salts are regained from the water and from their food. So, if you are using good aquaculture fish food (not gimmicky stuff), then you may not have to worry about increasing your salt levels.
The chloride component is actually very healthy for use as a preventative. This helps to aid proper body function of your fish and defends your fish against nitrite absorption. This is achieved by using calcium chloride (@ 16 ounces per 1,000 gallons, first dissolved in a small bucket of water then slowly release solution around the pond's parameter). The Calcium is also very beneficial to your pond's system and helps the fish to block toxic metals from entering their body and controls leakage of salts from their body. Many people do this once there is a decrease in the GH test readings or when there is a climb in Nitrite numbers. If this dosage is ever done, you really only need to do it once for hard water (or twice if you have soft water) since it is a quite high dosage.
Sodium salts are mainly used if the fish is going to be treated for bacterial infections (which is preferably done in a quarantine tank rather than pond if possible) and used if ever going to transport the fish. During fish transport, the fish actually loses more of its mucus layers due to stress, constant rubbing, etc. So, when the mucus layer is reduced, higher sodium levels becomes an irritant to the fish causing to regain this mucus layer so to help the fish maintain an osmotic balance.
Often times in aquaculture for food fish production, they maintain a 0.8% concentration of sodium chloride in the water since this most closely matches the salinity of fish's blood. However, they are also in a more "fish crowded" environment where fish are more likely to rub, lose mucus layers, and higher potential of bacterial infections (fin rot) due to frayed fins created by other fish. So, this gives a good benchmark where the "high end" threshold would be, but this should not be normal for our ponds.
Goldfish and koi are considered to be "stenohaline", which means they can tolerate only a narrow range of salinity change; however, the point remains the fish can safely tolerate a change in salinity. Do not forget... they are freshwater fish... so, just take quite low salt percentage levels.
Depending on where you live and where your source water comes from, the water salinity can range from anywhere 0.02% (that is .2ppt) to 0.18% (1.8ppt). So, many folk might not have to increase the water's salinity at all. This is the general area where salinity should be for our ponds, that is anywhere between 0.02% to 0.18%.
@Gemma , I hope this helps.
