Thank you guys very much....

I've thought about the parasite thing a bit... and I suppose it is difficult to say. Almost all of the ponds in FL abut some kind of cared-for lawn (especially with all the golf courses etc. in this area), so I was leaning toward pesticides/fertilizers in the water. But why, then, the proliferation of minnows in this particular small pond? I've also seen water snakes living in the same body of water... and besides which, my Florida blue is expanding in size by the second on these minnows (though I'm worried about him now too considering how random his cage-mate's death was!). And I thought I was doing them a favor by catching wild minnows rather than feeding the store-bought stuff...