Used to be that adding a tiny trace of salt (like, 1 tsp per 50 gallons or something) was recommended. It's perhaps not all that bad an idea for large tanks, as osmotic pressure (the difference in a fish's internal pressure to the pressure of the water they're in) can be improved by the addition of a little bit.
But the whole 'salt prevents disease' thing is kind of hit and miss. What you may end up with, depending on how much salt you're dosing and what your water change schedule is like, is a tank with a dangerously high sodium level in a few months, and/or salt of being of no use to you as a treatment, as whatever your fish has will be clearly resistant to it.
IMO it's better, in little betta tanks, to leave salt for:
- the first week of a new pet/LFS store fish in its new tank (I do this to head off any infections/parasites/etc the fish has been exposed to in its manky little store cup, as well as relieve some the ammonia and nitrite damage these fish usually are suffering) after which I do 100% water change and use just very clean water.
- clear signs of the beginning of a disease with which salt can help (fin rot, infection, ich -- not swelling, bloating or internal disorders that are better treated with -epsom- salt and not advanced disease best quickly treated with an appropriate antibiotic or antiparasitic medication).
A better (and unharmful) preventative addition is ketapang (Indian Almond Leaf) which does wonders for betta health by acting as a mild antifungal and antimicrobial, as well as making the water nice and tannin-y, which bettas really like.
Last edited by Aus; 08-23-2012 at 01:34 AM.