The first proper salt mix can be carried out as part of the brand new set-up. When the filtration, rockwork, substrate and heating is all in place, the tank can be filled with freshwater. Accurately mark out a bucket to say, one and a half gallons, and use it to fill the tank, making careful note of how many buckets are required. It is an easy task to then add up the buckets and assess the nett gallonage (the true amount). If the gallonage is totalled as the tank is filled, the side of the glass can be discretely marked at convenient intervals. This will make future water changes far easier to calculate and any medications/additives may be used with greater accuracy.
Fill the tank to within a few inches of the top, leaving room for the salt, and turn on the pump(s) and heater(s). Within 24 hours the water should be up to temperature (77°F - 25°C ideally) and the salt can be added. There are two important reasons for doing it in this order; firstly, salt dissolves far more quickly at operating temperature and secondly, aquarium hydrometers are calibrated to give an accurate reading at this temperature. Add the salt slowly and it should dissolve quite readily. A good tip here is to buy a packet of salt that is just over the estimated nett gallonage of the tank. In this way, after most of the packet has been introduced, a hydrometer can then be used to measure the rising specific gravity (amount of dissolved salt). Introduce the salt more slowly as the desired s.g. is approached (usually 1.021). Stop at 1.020 and let the solution become fully mixed. It will be noticed that the s.g. rises slightly as all the particles dissolve and might even reach the target. If the tank water falls short of the desired level, then add some fresh water, followed by more salt, until it does.
Two weeks after the tank has received its first occupants, a water change will be necessary, say for example, 5 gallons. If the tank has been marked, 5 gallons can easily be siphoned out down to the right mark; but first, the new water must be prepared. Always use a suitably inert container to mix up the new water. A glass or acrylic aquarium would do, but most people use a winemaking bucket, either a five gallon or ten gallon bin bought at a home-brew shop. These are ideal as there is no possibility that toxins from the plastics dye will be released into the saltwater and poison the livestock. Mark the gallonage accurately on the side if need be (rarely have I encountered the printing on the side to be precise). The procedure is much the same as mixing in the aquarium but can be speeded up if desired by adding freshwater (not saltwater) heated in a kettle until the correct temperature is reached. Add salt slowly until the desired s.g. is correct.
Alternatively, a heater/stat can be used to bring the water up to temperature over a few hours. An airstone will be needed to agitate the water and aid mixing. Check the temperature with a thermometer but do not remove the heater/stat until it has been disconnected from the mains and the element is cold (if removed when hot, the glass housing will shatter!).
Once the newly mixed water is ready, it is a simple task to remove 5 gallons from the main tank, discard it down the drain for it is of no more use, and replace it with the fresh.
Hydrometers come in two common types. The swing-needle variety and the glass floating 'stick'. Both are adequate and easy to read. The glass type may even possess a very useful in-built thermometer.
Water changes are an essential part of marine aquarium husbandry and it is easy to get into the habit of doing it regularly. Here is some useful advice :-