If the fish are staying in the pond, use only very low doses of formalin and be aware there is a risk of killing the fish along with the Ich. Formalin should not be used if you suspect the fish have gill disease or breathing difficulties. The recommended dose is 0.015 ml of formalin per liter of water. Every day for 3 days, partially remove some of the water and replaced with treated water. Encourage the air to circulate in the pond water as much as possible.
1500 gallons is equivalent to 6,810 liters. This means adding 102 mls of formalin to the pond.