Too many variables to give an answer. Wal-Mart might have some of their pool thermometers on sale. I bought one for about $6 during the summer they might be cheaper now. I also purchased an indoor/outdoor thermometer from them or Lowes for less than $10. Knowing the temp is better than guessing.

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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).