Are you aerating the newly mix saltwater?
This is important because the coral pro salt contains buffers as opposed to just raising the concentrations of alk, calcium and magnesium etc .
buffers need good mixing and a good amount of air to be properly activated. Adding the newly mixed salt H2O without giving the buffers time to activate can cause tiny fluctuations in Ph etc causing a teeny tiny bit of die off of bacteria, raising nitrate and phosphate a tiny little bit enough to start a tiny bit of cyano.
I always thought carbonate and bicarbonate were buffers!?