No, you don't drain all the water. Only 10%, so 1 gallon of water. I then replace it with 1 gal of distilled water, some people use RO water also. Straight tap water is not recommended for marine so I have been using distilled with great results.
You would also need to add more salt to maintain your desired reading. But when topping off you don't add salt. Salt does not evaporate only the water that is why it is very important to make sure your evaporation rate does not get to low before topping off. Your salinity will sky rocket on you causing a whole lot of problems.
If you have a 20 gal tank, I would strongly recommend that you use that instead of the 10 gal. Don't want to discourage you from the 10 but you would be able to put a bit more into a 20 gal. I have a 25 gal but not ready to part with my freshwater setup and all the live plants I have. Maybe oneday.
With my 10 gal I would slowly like to have more coral than fish. So as my fish die off I won't replace them but still love my clown fish very much and would always have one of those in there.
Everything is fine in my tank, but if I had to do another one I would probably put in one clown and one firefish or two of the perc clowns.
Do lots of reading on this forum and ask alot of questions also.
Good Luck
Debbie