105 ppm? That is extremely low and if that is right, that may be causing problems for your monti cap frag.
105ppm = 5.8dKH = 2.1meq/l
You should be at 180ppm or 10dKH or 3.5meq/l
You can use Arm&Hammer baking soda to raise your alk. Use this Reef Chemistry calculator to figure out how much you need to dose.
In 50g of water, you would need to dose 5 teaspoons of baking soda to riase alk from 5.8 to 10 dKH. You can mix the 5 teaspoons in a quart of RO water and add that over the course of 2-3 days.
Your corals and coralline is using it.Which is what their supposed to do.And the more hard corals you have,the quicker its used.Then the mag,calc,and alk. are all connected.They'll try to balance themselves out.
Use that link to the reef calculator to figure out the dosage,and use epsom salt for your mag.