Are they puppies then? If so, they could easily have coccidia or giardia. Giardia is notorious for hiding under the scope - so you often have to test twice for it. It can kill a puppy, so you may want to bring another fecal sample to the vet asap. Even though energy levels are good - what can happen w/ puppies and diarrhea is that one minute they're fine, the next, it's an emergency - I've seen it happen and it's incredibly scary (and expensive at that point).
As an aside, if it were me - I'd get them off of the Iams asap - it's really full of fillers and allergens. If you transfer them off early - you'll avoid potential allergies later on.
