I've added a few sps frags to my tank over the last month or so and I notice my calcium levels have decreased . The last two times I tested they were at 350 and I've had one reading of 320. I change about 15% (considering the water in the sump) of the water weekly. I want to get the calcium level up to around 400 or more if it won't harm the specimens. What are the different methods I could use to increase the calcium levels and how do each work? Also, how many of you add supplements to your tanks?