h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Another way to treat your water that I discovered recently is to use aquarium salt. You need very little: I only put a pinch into a one gallon bowl. It's much more difficult to overdo it with aquarium salt than the harsher chemical treatments, and the salt is much cheaper. I have a betta; they're freshwater fish but a little salt in the water works wonders. It cuts down on bacterial growth and apparently gives them a lot of minerals that they need. It also seems to help soften the shock of a water change.

Keep in mind that aquarium salt is not the same as table salt! Table salt is over-refined so it has lost a lot of the minerals. It's also often got additives like iodine that harm the fish. You can buy aquarium salt in pet shops or anyplace that sells pet supplies.