Like most rules of thumb, there are exceptions. A white perch is designed to feed up. Doesn't mean they always feed up. That being said, when the perch are suspended in deep water and you are fishing below where the fish are holding, you are in for a LONG day. Always best to fish higher in the water column and work your way down til you find them.

Being full of crawfish doesnt mean they are feeding off the bottom. Those crawfish get in the floating grass too.