i would hold off feeding for a few days to minimize waist build up. as for the water change im not sure, you just did a water change and i never used prime so i dont know how much nitrates it will get rid of... danh would know.. i hate to tell you to hold off and wait for danh because the longer you wait the worst off you might be.. how bad do your fish look? are they gasping for o2 ect? add an airstone for now if you have one... i know nitrates take the o2 out of the water and pinch fins