Alkalinity dropping more than normal

Hello guys and good evening. In the past two weeks i’ve been having issues with my alkalinity, i normally keep it at 8-9 dkh but suddenly it started deopping getting at 7 or even 6 dkh( i do water changes every week on my 32 gallon tank and i mix my water using red sea coral pro. ) this change bleached my sps, i was able to bring them back but my alkalinity keeps dropping about 0.5 dkh every day. My magnesium is too high 1480 reading by red sea mg pro test kit. And calcium is always at 460-440 ppm. Is this normal based on that i have 4 sps frags that are growing real quick after I upgraded my lights. Do they use more alkalinity than calcium? I changed all my filter media at once, can this cause swings in my alkalinity? Please share any
experience that you had and for the salt. Do you think i should use another brand with lower