Helping my frogs survive this cold, snowy winter

Unfortunately, our frogs haven't survived the last two winters. Fortunately, they had left tadpoles that did survive so we were able to enjoy frogs each year for the past few years. We've tried to come up with a way to help them survive in our ponds without the use of electricity. Our attempt this year is to sink PVC pipes into the pond so that there's a source of oxygen in the pond. I check it each morning to make sure the water in the pipe isn't frozen. If it is, I pour warm water down the pipe and that takes care of it. Surprisingly, even on very cold days, it generally doesn't freeze, so I haven't had to thaw it very often. I worry about adding warm water and raising the temperature in the pond, but since I use less than a gallon, I don't think the warmth extends too far into the pond. I have no idea in which part of the pond the frogs decided to overwinter, so I don't know how close they are to the source of the warm water. If this method doesn't work 1) we'll have to acquire some new tadpoles or froglets this year (easy to do since people get them in their swimming pools and they would otherwise die when they add chlorine), and 2) we'll have to give some more thought and research to our overwintering strategy. We hope to be able to devise something that doesn't require electricity, though.