SPS bleaching

Good morning guys, it's been three weeks that I've been struggling with my SPS frags. I am that guy with the 32 gal biocube that have asked like 2 million questions :). I just don't know what to do,I upgraded my lights to ai prime hd, I stopped making water changes for a week an I've kept all my parameters stable. alk 8dkh, calc 440, mag is always high because of the salt I use at 1420, 0-5 nitrates and phophates are always like 0.25 it gets lower when I do water changes. At this point I don't know what to do only one frag has died, the rest they are ok. I picked up a Birdsnest frag and it has bleached 50% in a week and it started since I put it in the tank. I have my ai prime with the purple and blues at 119% cool white - 19% and uv is high. I downloaded the setup from BRS. the only changed I made was the filter media all at once( Chemipure, purigen and I added more matrix filter media) good to know that this problem started happening after this change and with my old biocube hood lights and it hasn't stopped it is frustrating. I don't like small tanks but I don't have more space at this time for a bigger tank. honestly guys anything can help. This is just affecting the sps I have all tye of corals that are doing amazing