L.Mack Welcome aboard. I would suggest a representative water sample taken and test for the obvious,PH alkalinity,TDS No3 P4 etc. Then see what your options are. Simply tossing alum at it may be a "shotgun" approach not needed or wanted from a PH standpoint.I would also address the aeration you have been using as that may be part of your problem .I assume you ice up for several months per year. Lastly what type of fish and size (age) have been winter killed in the past.
