Active ingredient: API Aquarium Salt: salt made from evaporated sea water (sodium chloride with other sea-salt electrolytes); exact composition not stated on the product page
How to use (from the label): 1 rounded tablespoon per 5 US gal (20 L), or 1/2 rounded teaspoon per US gal (4 L). Salt does not evaporate or filter out, so add it only with replacement water at water changes.
Careful: Some live plants may be sensitive (use care, not a ban). Some fish do not tolerate salt well, notably some tetras, and fish that navigate by electric field (e.g. elephantnose) should not be kept in salt.
Never mix medicines unless the label says it is safe, and remove shrimp and snails unless the label says they are safe.
Examples you may see in shops: API Aquarium Salt. Not endorsements. Always follow the label.
5 facts checked with sources