Performing regular water changes are fundamentally important to maintain the overall water chemistry inside of a closed ecosystem. The frequency on which one needs to do a water change on their specific tank will vary based on size, bioload, and other factors - however it is a required aspect to make sure that water chemistry is balanced as it will fluctuate through time.
Water changes are mostly required as they remove nutrients while also allowing for some nutrients to be replenished that are consumed or transformed. In the beginning stages of your aquarium, it may be the only way elements are replaced. If your aquarium is looking dirty, or your corals are not as vibrant or acting as nature, these are key indications that a water change should be performed. In other cases, if there are high nitrates or phosphate being shown when using a test kit, one must perform a water change to lower those levels down.
We have to consider the inhabitants of our tank when we add new water in, by not only making sure the temperature of the water you are adding is the same as the tanks current temperature, but to also make sure that is is done in increments. In a freshwater aquarium, one can simply use tap water that has been dechlorinated - however for a saltwater aquarium, it is highly important to use RO (reverse osmosis) water to not add in any nutrients that may not have been in the tank already (or to disrupt other elements that may be not really useful overall). Test the salinity of your new water so it matches your current levels, as if the salinity gets either too high or too low it can be very stressful for the inhabitants that you have. You make saltwater by mixing RO water, and aquarium salt, which you can purchase at a local fish store or online. Most aquarists tend to keep large plastic drums in their homes for mixing saltwater as it allows for an easier way to measure and adjust the salt or water level as needed to find a level that is extremely close to your current tank.
Once your new saltwater is ready and you have verified temperature and salinity, you are ready to start removing water from your display tank. This may also be a really good time to clear your aquarium glass or clean any algae or debris that is starting to pile up in a stale spot on the substrate/rocks. If you are scraping algae into the tank make sure to suck it up with your siphon to not allow for it to spread around the tank and end up forming in multiple new spots. Keep removing water until you’ve reached the point that you want (normal water changes are between 10-30% of the water at once). If you have corals or any type of live rock near the top of your tank which could be exposed to air while doing your water change, make sure to turn off any lights to avoid drying the area out too much or burning it with the lights on them. You will want to quickly then refill the tank with the water that has been prepared ahead of time in order to reduce the time the corals/live rocks are exposed. Many tend to add the new water to their sump and allow for the system to fill it up, otherwise you can pour the new water slowly over the top of the aquarium using either buckets or a pump to allow for the water to be cycled through the tank.
Once this is done, you are finished with your water change! You can now take the buckets filled with the old tank water and dispose of it either in your backyard, down a toilet/sink/tub, or anywhere else that water can be poured into. We recommend that you start off by doing a water change about once every 1-2 weeks, and adjust as needed based upon your specific tank and the signs that it gives you when it's time to add more fresh water into it.