Bass,
You might want to consider keeping a half dozen steralized water sample bottles on hand... the kind used for well water testing would work fine.
If you go the the lake some day and find a mass of fish belly up take several water samples, using latex gloves for your own protection as you have no idea what this guy might put in the water.
If there is an inflow to the lake, a creek or stream, I would also try to get a sample of the water coming in to the lake (without tresspassing on his property if it is on his end)to show that the problem did not originate upstream. Testing of the samples (labled carefully) should be able to show what chemicals were used. Around here the EPA and other agencys have 5 working days to respond to complaints and they usually take at least that long.
You still would have an uphill battle to prove he did it but maybe he would be dumb enough to leave the empty containers around or some other evidence you could use along with his statements which would be better if they were made in front of witnesses.
The next move would seem to be his but I would be prepared if he carried thru on his threats. And you wisely noted that if you beat his butt you would probably end up in court for assault and possibly even sued civilly.
Good luck,
Dan
