I fish two spring fed ponds that are in between a small creek and a good sized river. The gills are always eating bug on the suface. Mostly caddisflies but also mayflies, I don't think I've seen them eat any stones but Iam sure they do. There are stones that hatch on both the creek and the river and I have caught fish on both waters with stonefly patterns, so Iam sure they reside in the ponds also. Just yesterday I had very good results on one of the ponds using a generic parachute pattern, the gills were breaking the surface all over. Iam sure you could use any stone patterns typically associated with trout fishing and produce some gills. Hit the shoreline and flip over some rocks and logs in the water, you should be able to see whats going on down there. Try some dragon/damselfly nymphs too, their in there and can be very productive. Good luck and keep us up to date on how you do!