temper chaenj=52 deg F, so yu need 52 Btu/pound . . . wen ies melt, it absorb 144 btu/pound . . . 52/144=0.361111111 pounds ies per pound water... . . water at 32 deg F: density=62.42 pounds/kubik ft or 8.34 pounds/gallon . . . 70 gal wate=583.8 pounds . . . ies yu need=0.361*583.8=210.8, or bout 211 pounds