It's a very bad idea to add salt "once a month".
Salt is only removed by water changes. If you want to maintain a certain level of salt (really only useful in treating illness), you need to add a given amount per gallon initially, and then only add enough salt to replace what you take out during a water change. For instance, if you want to have 1 tsp per gallon in your 20 gallon tank, you would initially add 20 tsp. Then , when you removed 5 gallons for your water change, you would add 5 tsp of salt. Now, you might end of putting 6 gallons back in, but remember, that extra gallon is due to evaporation, and salt doesn't evaporate out with the water.
All that being said, adding salt long-term isn't going to help your fish any. "Salt" is a very generic chemical term refering to a wide varitey of compounds. It's true that the Rift Lakes do have a higher concentration of some salts in them, but that's not the same thing as having a higher concentration of Sodium Chloride (NaCl), which is all most "Aquarium Salt" is (same as table salt). Now, you can purchase salts that mimic the salts in the rift lakes, but those mixtures are really more useful as pH buffers. If you have soft water I'd reccomend it, but not otherwise.
