if you are using tap water, 2 changes a week arent going to do anything because youre still pouring impurities into your tank. tap h2o is loaded with phosphates and heavy metals. Phosphates inhibit coral growth and can be the cause of youre issue. i would invest in a nice corallife ro/di system first before wasting anymore salt.
youre not taking any of my time i love helping others, keep asking questions if you want.
i used to be like you using tap water but i learned my lesson, take it from someone that has done it before and get that ro/di system. theyre only 100-200 bucks! not too bad when u compare it to the amount of money youve lost in corals...