Little Giant® Trough-O-Matic® Stock Water Tank | Float Valve Controlled Watering Tank with Plastic Housing (Float Valve, 1, Count)

Probably just evaporation that is happening from summer in the sun and depending on your pond surface area, low humidity, lack of rain, if it is windy, etc can all contribute to loss of water. For you gallon size, about 1 to 3 inches or 7 to 25 gallons per week can be expected. I use an automatic float water leveler to keep the water level at a particular level for optimum skimmer height... Something you may consider so you are not having to constantly top up pond always. With auto water leveler, also another great option is an overflow if the pond were to ever get to high, say from a storm. One last thought, when you do top your pond up with local utility water, it is good to also add some de-chlorinater for water health.