20 to 25% should be more than enough. u can forget abt dosing additives too if u r doing weekly water change....... that is if ur corals do not take up all the necessary elements within 3 days!!! which i doubt so.
IMHO, as long as the SW u buy is safe and in good condition, i dun think there will be any problems........ but how much of ur tank volume u change per week..... that's crucial.