Yeah I'm all mixed up...I thought "spslvr" was OP. Sorry St Patrick...some of my response was from what spslvr had responded to me.
St Patrick, the pic you posted looks like response from alkalinity swing from it dipping to 6 dKH. (The high alk comment was for spslvr).
Also, FWIW, alk that low won't cause pH to get so low that the sand will dissolve. You have to use CO2 to get pH to about 6.5 to dissolve calcium carbonate. pH of 6.5 not alk of 6.5.