Not only do guppies and neons co-exist quite happily but the guppies give the neons alot of exercise. If the Neons aren't right smart about darting up to get some sinking food the guppies will scarf the lot on them. So I have very fit neons! Another benefit is, Neons LOVE snacking on guppy fry, so the guppies provide them with live crunch n' munch. Let's just say you won't end up with any slow or stupid guppy fry.