Does it have a shelf life once mixed?I mixed a gallon of BRS Mag about a year ago. ?In the meantime finished using my Kent M. ?Couple of weeks ago I noticed mag really low (1170). ?I started dosing..Mag dropped a little lower. ?I then upped to dosing 50ml twice a day on a 90g. ?Checked again last night it was at 1100.I think tonight I'll try a different test kit. ?Although this one is not that old.