They likely behave in nature the same way they do in tanks. In SC we have a fish called "crappie" (I have no idea how it got that name but they are good to eat!). Pomoxis nigrolineatus. They have a VERY upturned mouth and you'd think "surface feeder". Oddly, you only catch them in 10 feet of water or more (50 feet in the winter!). The trick is to get the bait just over their heads. They swim low, looking up for a meal swimming overhead. I'd assume that neons are a nano-version of this, feeding on planktonic stuff floating overhead.