First of all, you will need a bucket/container to store the water in so that you can prepare the water change a few days in advance in order to let the salt dissolve. You must make sure that the bucket holds the right amount of water for the amount you are changing.
Fill the bucket with the required amount of water, then add a dechlorinator. Read the instructions on the bottle to know how much to add to the volume of water you are using. I use Nutrafin Aqua Plus but there are others such as Seachem Prime. You should add the dechlorinator in advance so that it has time to take affect on the water.
After you have added the declorinator, you must add marine salt. After this, you should add an air pump and a powerhead (optional) and heater (optional). The air pump will help dissolve the salt, the powerhead will help circulate the water and the heater will keep it at the same temperature as the tank.
I leave my water running for 48 hours before the water change so after 24 hours, use a refractometer/hydrometer to check if you have the right amount of salt. So you will have time to adjust the salinity. If the salinity is too high then simply empty out some water and add fresh water. Remember to add a dechlorinator because even though you are leaving the water with an air pump for 24 hours or more it will not get rid of the chloramine. I am aiming for 1.005 since I have figure 8 puffers. A refractometer will be more accurate since hydrometers are temperature sensitive.
After you have done all that, you are now ready. You will need to get another bucket to syphon out the water from your tank to the bucket. You should empty out about 50% of the water.
Once you have syphoned the tank, you must clean the filter in the tank water you emptied. If you were to clean it under a tap, the chlorine would kill off the living bacteria on your filter media.
Now it is time to get the bucket that you used to prepare the water change and empty it out into the tank.
Please remember, water changes should be done once a week.
