I'm guessing that you don't have a bottom diffuser aeration system in the pond. That's exactly why those systems are recommended for ponds - to eliminate that type of water.

That's normal for a pond that doesn't have one. Maybe not the black color, but the smell, yes.

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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).