If you drop into a pool supply place and ask where they get their bulk chemical they may be able to hook you up with product or even the supplier.

I do think even if you do everything correct you need to watch the EC of the pond water. If high to begin with then adding alum and hydrated lime the EC could go much higher than the fish or life can handle. If I remember correct and EC of 16 is ok but get to 24 and things die.

EC is electrical conductivity of water. It measures dissolved salts.

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7/8th of an acre, Perch only pond, Ontario, Canada.