Spark duration if you need this to be adjustable then an ignition coil might not be the way to go, ignition coils are flyback devices, the turns ratio is usually only 100 : 1, so to drive a continuous spark you'd need a very high input voltage, an ignition coil discharges its energy in a similar way to a marconi oscillator and controlling spark duration might be tricky.
You might be better off winding your own tranny, something like an Etd25 or something like that.
How do you 'tune' a supply to the pond, does the mass of water have a particular resonant freq?
You could be correct as far as the final stage transformer is concerned. I'm going with an automobile ignition coil (at least at first) because the newer ones (still pure flyback) put out 25-40 KV, are fairly robust, are sealed, and cheap (got 2 off ebay for $5 each).
This high voltage stuff is new to me, as my only experience is with control circuits/software (mostly software)...
(unless you go back to my navy radar dayz, 40 years ago, and i was a tech then, not an engineer...)
If it ends up requiring me to wind a custom transformer, guess I will. But only if required...
As far as the "tuning" goes, yes, you DO tune the signal to match the pond water. This is according to the Mississippi Agriculture Department, which has the more expensive equipment which can do so. Different ponds contain water from different sources (more or less iron, calcium, etc...) and also contain different types of fish, runoff (watershed, soil, manure, fertilizers), and contaminants (trees, dead animals, fish food), which can greatly change water resistance and/or "resonance". (the ag guys are farmers and don't really know much (like me) about their equipment, but they do know about how different ponds require different settings to work)(wish I could just get them out here)
Most of the fish zappers available for sale out there, actually state they only work in flowing water, specifically NOT in ponds...
(ponds retain heavy sediment which cause my problems...)
Your insights are why I started this thread.
Thank you very much...
william...