Question about 20% water change

I was curious about something...first off I'm very new to the hobby but anyways I know it's recommended to change 20% of the water every two weeks but I am curious how much salt mix should be added to that water,for instance I have a 46 gallon aquarium

I'm new and just worried I might do something wrong so any helpful insights would be so much appreciated

 

The water used for water changes should be the same salinity as your tank (assuming your tank salinity is at the level you desire - I keep mine ~1.026). This usually equates to around 1kg salt for every 30L water (2.2lb per 7.9gal - sorry I only know metric units), however the salinity should be measured using a hydrometer rather than relying on weight/volume measurements.
For your 46gal, a 20% change equates to ~9gal water with ~2.6lb of salt.

The water change is to replace some of what the corals/fish have taken from the water, and to remove some of their waste products.

Pure water should be used to top up evaporation.

 

Who said its best to change 20% every two weeks? That's an awfully aggressive schedule. What are your water readings between these changes? I personally don't do any water changes at all, and my systems are fine
Anyways, when you do a water change, everything should be done in buckets before you add anything to the tank. Your salt mix should have a measurement on the back of it, per gallon of water. That will get you close. Put the water and salt in the bucket, throw a powerhead in there, and let it mix for at least a half hour, or until all the salt is dissolved and the water is clear. Check it with either your hydrometer or refractometer depending on what you use, then you can add salt or more water to match the salinity of your tank. If you have a fish only, you should be around 1.018-1.020, reef I recommend around 1.025-1.027. When everything matches what was previously in your display tank (including PH and temp) take the bucket and dump it in. Really, you should have the water all ready to go before you even start removing the water from the tank.

 

+1 to ^^^

Water changes are done either to import elements or export nutrients. You should be testing your parameters and only changing water as needed. There is no one fixed schedule that's right for every tank.

 

Also, when you mix your water, it is always a good idea to let it sit for 24 hours prior to adding it. This way the salt will be completely dissolved and you won't have to worry about adding the water and then ending up with higher salinity than what you initially measured.

 

Oh wow thanks everyone all of your replies are very helpful and definitely give me a heads up on water changing.

I have heard no two saltwater aquariums are alike I do think we all probably do everything to tailor to our aquariums stock needs

I definitely like the idea of premixing and using a powerhead in a bucket to mix it and waiting the 24 hr period before using it,this is some very helpful information.

 

Usually salt mixes will have per-gallon directions. One cup of Instant Ocean salt makes 1 gallon of saltwater. So 20% of 46 gallon is 9.2 gallons. So lets say you do a 9 gallon water change, you want to put 9 gallons of water into a separate container, than add 9 cups of salt to it.