The rule of thumb is 15 min the first day, 30 the 2nd, 60 the third, etc. But if the smell is strong, I'd go slower, maybe 2 days at 15 minutes, then 2 days at 30 minutes. Hydrogen Sulfide is pretty toxic to fish, even in small amounts.

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).