I keep the big heater away from the liner, and in the middle of the middle pond. I don't know why my ponds seem to do good by just toping off the ponds. Things seem to work and I believe that the benificial bacteria will convert the nitrites to nitrates, read that somewhere long time ago, and then the plants can obsorb the nitrates before they become harmful. You need to have a well established pond with lots going on, plus I do have an overflow that takes water out when it rains hard too. During the winter I do top up the pond a lot, water evaportes, and then there is water displacement caused by excess ice build up. The odd time I forget that I am topping up the pond and then the water overflows for a while out the bottom pond too.
I am sure some water escapes here and there behind the water ways, that is possible too, whatever the reason, things seem to work out good for me with no water changes for years. Now I would not get away with that in a small pond, or in my fish tanks either.
The only time my ponds ever got a good water changes was about four years ago, I re-dug out the bottom pond and had to replaced the water in that pond cause I made it bigger. Then about six or seven years before that, I remade the middle pond and that was another big water change too. Then about 15 years back I re-dug out the top pond and made that bigger too. Before that, about 18 years ago, there were other changes too, too far back to remember anymore. In the 23 years of having ponds, the only major water changes were when one of the connecting ponds were re-dug or made larger. I don't know why it works, maybe I add more than I think, but for some reason things seem to do well, and my two oldest koi are about 24 years old now.
