It takes several steps, and you don't FILL the tank each time. Put just a few gallons into the tank via the pumpout fitting--'cuz that sends the water into the tank at the bottom to stir up any sludge, which putting water down the toilet can't do cuz water from the toilet comes into the tank at the top. Pump that out. Repeat...repeat...repeat...until you're pumping out clean water. Then, if you have an overboard discharge pump, put a few gallons of water into the tank one more time and turn on the pump to rinse it and the overboard discharge plumbing.
Or, if you have a washdown pump AND can take the boat out past the "3 mile limit," you can use the washdown pump to add a few gallons of water to the tank, then turn on the overboard discharge pump while you let the water continue to run until you no longer see any cloudy water coming out of the thru-hull. When you come back in, stop by the pumpout and run a few gallons of clean FRESH water through the system to rinse out the salt and sea water minerals.
You only need to do a thorough rinse 2-3 x season and especially before any extended layup, but it's a good idea to put a few gallons of fresh water into the tank via the pumpout fitting once after every pumpout. And you'll have fewer problems with a macerator pump if you'll run plenty of clean water through it any time you know it will sit for an extended period before it's used again.
