diablo30xp

I just recieved my hanna phosphate checker (HI713). Tested my water the way the instructions said. Is it possible to have a bad reading after 3 tries? I have green hair algae growing and algae on my sand bed growing for about a month or two. Will it show any readings to measure? i want to make sure im doing something right or not. Ialso bought some phosphate rx should i still dose some to see if i get any readings after the fact?