Originally Posted by

**RobF** A pond (and filter) is a system (just ask Steve Childers). This system can be described by a transfer function and hence can be described by an equation. Transfer functions are in the realm of Electrical Engineering, and the Filter Theory therein is pretty well understood, defined in the time domain and applied through the frequency domain. Engineers of the various types, Mechanical, Civil and whatnot have adapted Filter Theory to apply to their applications, with some success, at least in theory. In the case at hand, koi pond, the good news is that the primary forcing function is the water flow. The pumps, pipes, bends, valves, all have simple electrical engineering analogs, and in the simplest case one could even regard flow as steady (and a constant in the frequency domain). The levels of particulates might also be regarded as constant. However ammonia will fluctuate with feeding and is probably triangular or sinusoidal. And the bacterial response is likely some kind of exponential. The efficiency of the filters will vary as they foul. Waddington says that koi can never be a science. I am sure it could be, but science is kiwa not sashi.