If you have a filter that is the proper size for your pond, and clean it out on the right basis, the simple emptying of the water in the filter and re-filling should be enough of a water change, no?
I don't understand the claims of toxin buildup of phosphates and proteins if you have a well-filtered system, and you are doing the above emptying of the filter.
Additionally, the proper oxygenation of water through waterfalls and other methods should additionally help with release of certain 'toxins'.
While it is true that solids, for the most part, remain in the water, the natural system of a pond is being ignored. Salts, phosphates, proteins and everything else typically has a counterpart that uses those items (just like the natural balance between animals and plants with oxygen and CO2). Additionally, you can add counteracting minerals versus doing a large water change.
I can't understand a 20-25% weekly water change. Let's take this to a much more closed system - the fishtank aquarium. It is not advised to change that much water at single point unless there is something significantly wrong. Before beginning ponding, I've had aquariums since I was a little boy and, as long as you have a good balance in your tank, there is little reason to do water changes more than every couple of months - and that is usually only in conjunction with vacuuming up all the debris that has accumulated in the gravel at the bottom of the tank, changing filters, and so forth.
As it is, I sometimes go over a year without touching my aquariums with no ill-effects - no diseased, sick or dying fish, and crystal clear water.
I've taken the same approach with my ponds - though I do tend to them a bit more often to clear away leaves, plant matter, and account for evaporation and a small leak I have not been able to track down for the life of me.
No fish problems to date over the past 3 years.
