Low levels of salt concentration are also used by fish keepers who maintain their fresh water fish in tanks or ponds where no biological balance is attempted. The low concentration of salt will help keep the fish healthy, warding off disease and infection, and keeping their slime coat in top condition. If the pond is equipped with biological filtration and some plant life, then there is no reason to keep a constant, low-level of salt concentration in the water. In fact, many plants will not tolerate salt in the water. Pond salt is also increasingly available, designed especially for treating illnesses in fresh water fish such as koi and goldfish.
Most sources recommend that fish be treated with non-iodized salt.
Iodized table salt is frowned upon. There is a concern that long-term exposure to iodine can be toxic to koi and goldfish. In the field, I have not personally seen any evidence of iodine toxicity from the use of table salt to treat fish. By the same token, I do not know of studies that have confirmed the benefit of using iodized salt to treat fish. Good judgment suggests it would be best to avoid the possibility of iodine exposure.