I think your suggestions are valid. On my ADF75, portions of the lower part of the hopper are coated with a tarry like substance, which I think they use for rustproofing and as a sealer. The actual bottom of the hopper is flat, the sides don't end at the opening. If I can, I'll take pics of inside the ADF75. (don't know what the roads will be like tomorrow - the unit is 2 miles away)

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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).