Like was said, check for alkalinity and hardness. If those numbers are too low, then fertilizing will be a waste of time - the lake won't be able to utilize the nutrients in there.

The clarity of 16"-18". Is that due to clay turbidity or a phytoplankton bloom?

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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).