I already posted this in the Salt thread, but I feel it is important enough to bear repeating. I am on a roll today.
It has already been beaten to death, but I will lay it out one more time clearly:
1. Salt is something that helps under certain circumstances (injury, some infections), but only under medicinal doses, which are much higher than most aquarists would use as a "tonic".
2. Salt used at medicinal dosages will mess up the osmotic processes of (non brackis) freshwater fish over the long term, and should only be used for short periods (a few weeks at most) for treatment of specific conditions.
3. It really does not matter what salt you use. Table salt will work, even if it is iodized. The iodine concentrations in table salt will not be harmful to your fish over the short term.
4. Some fish, particularly scaleless fish are particularly sensitive to salt and special care should be taken when treating problems with these fish. Generally, the dosages will need to be lower.
5. "Tonic" dosages, in other words dosages much lower than medicinal dosages used continuously, will not benefit your fish, and can cause potential problems if you are not very careful about keeping up with your dosing schedule. Salt does not evaporate. The only way it leaves your tank is through water changes. Keeping salt levels constant requires knowledge, recordkeeping, and consistancy. For these reasons, I strongly recommend against its use as a tonic.
6. In most cases, your tap water has plenty of minerals/electrolytes/etc. for your oscar.
