In a 150 square foot pond, a mere 1" water loss is 12.5 cubic feet of water,
or almost 95 gallons. I have a small 900 gallon pond with about 120 sq. ft
of surface area and a too-splashy waterfall cascading about 3 feet over
rocks which get wetted off to the sides. In the hot summers here in Memphis
I can lose almost 2" a week if I run the falls a lot (about 150 gallons, or
over 20 gallons/day). If I don't run the falls at all, I lose less than 1"
a week. When our power was out for almost 10 days from July 22-31 after the
bad storm here, I lost only about an inch, so that's a rate of about 7-8
gallons/day. The goldfish seemed to be totally unaffected by the stagnant
water, but the algae gained a bit on me...