ML,
Having zero experience raising trout, I will yield to LRunkle's post as it leads to some very useful information from Cecil.
On the issue of your sand trap, current stormwater/runoff mitigation practices during construction, in our area, involve the use of fiber netting. Prior procedures incorporated the use of hay bales. Personally, I would not use a sand berm for fear it may compound your problem during heavy rains unless it was planned as a permanent fixture and seeded, as noted in one of Cecil's posts.
Thanks for posting pictures of your pond. They represent a textbook case of the adverse effects of uncontrolled runoff/siltation. Hopefully your neighbor will take the necessary step to remedy the problem.
