How much water for a water change

I feel like this should be a easier thing to find online but I can't. I have a 90 gallon with a 200 gallon rated sump, a 60 gallon rated skimmer, a UV sterilizer rated to a 100. I have two clowns, one hippo tang(4-5 inches) a yellow tang, coral beauty, and a scooter Blenny. Also I couple muchrooms and other corals. How much water should I change? I just did a 10 gallon which I know is not enough. Should I being doing 20?25? All my chemistry is fine.
Ammonia 0.0-0.1
Nitrite 0.1-0.3
Ph 7.9-8.1
Its difficult to tell for me so I'm saying the two it could be.

I don't remember what my nitrates was at but it wasn't that high. This is a new tank so I didn't suspect my chemistry to be bad. I switched from a 45 to a 90 two weeks ago using all pre made water. So how much water 20? 25? 30? Anyone who has a 90 gallon with a high bio load have an opinion?

 

5% to 10% of your total volume is very common and should do you well.

 

Also can someone tell me what container they use to mix salt and where they got it if you do a large water change.

 

use a 5 gallon bucket and put a power head in it. I think you'd be using two 5 gallon containers of RO water so just do that twice. JMO
Home Depot sells 5 gallon buckets cheap if you don't have an old one from salt.