Calcium Issue

[Snufflupagus]

Hello fellow reefers,

I have a calcium issue happening in my tank right now, and have no clue what's causing it or how to fix it. My calcium keeps going up even though I stopped dosing it. I have a heavy stalked SPS tank. The corals are starting to bleach a little slowly everyday. I've done water changes twice now, I've changed over 40% of the water (150 gallon, 180 TWV) and the calcium is still reading between 550 and 600 on my Hanna checker. The test kits were just bought and have not expired. My Mag is 1350 and my Alk is 8.4. One day I'll do my test and the calcium will be at 520 so I think it's lowering and then the next day I test and it's up around 580 without having dosed or adding anything to the tank. I use Reef Crystals for my water changes.
Anyone have any ideas on what may be causing these levels or any tips and tricks on getting these levels lowered?

Thanks