how and how often do you adjust the salt level in an aquarium?

Answer 1

You should only need to adjust when you do a water change - this would be the same for either a marine tank, or a freshwater/brackish tank.

You should mix the saltwater outside the tank if the water has a high concentration of salt so you can get the salinity correct before you add it to your tank (in smaller volumes that are used in freshwater, this isn't as critical, but the salt should still be dissolved in water before you put it in).

As long as you have the correct amount in to begin, you only need to add water as evaporation takes place - the salt doesn't evaporate, but stays in the tank. This will increase the salt concentration (as the amount of water decreases), so you should top off the tank with dechlorinated water every few days. Note that it's the water level you need to be concerned with here, not testing the salinity - as long as the water level stays about the same, the salinity does as well).

When you do a water change for a freshwater tank, keep track of how many gallons you remove, then replace the amount of salt removed with it. That's why measurements of a tablespoon per 5 gallons or a teaspoon per gallon are commonly used, depending on they type of fish you keep. It makes mixing replacement water easy.

If you're dealing with a brackish or marine tank, you want to use a hydromenter (http://www.thatpetplace.com/pet/group/11318/product.web ) rather than volume measurements to get the correct amount of salt. For a marine set-up, you want a hydrometer reading of 1.020-1.016 for fish, or 1.024-1.026 if you have invertebrates. For brackish species (between freshwater and saltwater) you'd have to go by the salinity needed by the specific fish.

Answer 2

That really depends on how much water evaporates from your tank. Water evaporates, but the salt is left behind, so you need to top off with fresh water to keep your salinity from getting too high. If you have a sump, it's much easier to monitor evaporation. If you don't, you will just have to test your water everyday, until you get an average of how much you need to top off. Refractometers will give you the most accurate results, as hydrometers loose accuracy over time. When you do a water change, simply replace the same amount of water you take out, with the amount you put in, adjusted for the same salinity that's already inside your aquarium.

Regards

Answer 3

Check it every other week.
If you don't have, and don't want to purchase, the special tester for saltiness, take a water sample to a specialty fish store. They will tell you what's wrong (if anything) and sell you what you need.

Don't just randomly toss salt in there. Levels have to be pretty much perfect, and stable to maintain healthy fish.