I don't consider the run-off from adjacent yards to be fatal.

My reference to a 1/4 acre pond was to have maybe the best of both worlds. Have your own small pond to manage and grow big fish without molestation by strangers, as well as living on a bigger pond to which others have access.

The effect of run off of lawn treatments can be somewhat mitigated by a strong water source for the ponds.

When viewing potential place to buy, you should be able to see the condition of the pond(s). If they're choked with vegetation, maybe avoid those.

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."