Sorry to hear about your devastating loss! Like others have suggested, do the WCs and bring your params back in order again. Also, recommend to manually brush those white deposits and manually remove them via filter socks. This happened to my other tank where I dumped kalk & flooded the tank with RODI water. Luckily all softies and fish survived but I lost all the SPS frags.
I'm also running a recirculating CO2 scrubber (ph was 7.8 - 8.1, now 8.1 - 8.4) and always worry about this exact overflowing scenario! I've been using the AutoAqua Smart Skimmer Security as insurance but should look into a secondary method to dump the overflow back into the tank vs sucking thru the CO2 reactor...
Good luck and hang in there!