When keeping a saltwater aquarium, frequent partial water changes are necessary to maintain a healthy environment. However, you obviously can't simply use water out of your faucet without some modifications first. You need to make seawater as close to the existing water in your aquarium as possible. Luckily, you can make artificial seawater fairly easily in just a few minutes, and with inexpensive household items.
Fill a bucket with warm, fresh water from your faucet and let it sit for about half an hour, so it comes to room temperature. If the water's too cold or too warm when you pour it into your aquarium, it may throw off the overall temperature of the tank.
Add the proper amount of sea salt. This varies, depending on how much water you have. Different sea salt manufacturer's have different concentrations but, on average, you need to add about 118ml of marine salt per gallon of water, which can be measured using a kitchen scale.
Mix the marine salt around in the water until it completely dissolves. The water may be a little cloudy. Scrape the salt that settles at the bottom and mix it around so it dissolves with the rest.
Fill your hydrometer, which measures salinity, with water from the bucket by dipping it beneath the surface until the beaker fills, then place it on a dry, flat surface. Watch the floating needle in the hydrometer. If it points between the 1.0020 ppt and 1.0024 ppt markers, the water is fine and ready to add to your tank.
Never use standard table salt to make seawater. Only used marine salt specially distributed for the purpose of making artificial seawater, like Marineland's Instant Ocean line of salts.