You'll want to use a salinity Refractometer to measure the salt currently in your tank and ideally match it when putting in new water. It should be somewhere between 1.020 and 1.025 depending on what fish you have.

You don't need to take out the fish when cleaning, that will stress them out more. Leave them in.

Does the tank have coral? If not, then regular water changes should keep calcium, magnesium, alkalinity, at good levels. Testing the water every couple weeks is a good idea.

The main thing to keep in mind when cleaning filters is never use unconditioned tap water as tap water has chlorine that will kill beneficial bacteria in the filter. Other than that, just go to town with cleaning it. Nothing special, just scrub it, move things around etc until it looks clean.