It could be a young jack dempsey, with the red edge along the dorsal and the prominent spot at the base of the caudal fin. If it is, I'd expect it to be showing blue flecking on the sides and unpaired fins at that size, and that largest spot along the side should be showing a little more color around it as well. I can't tell if that's not showing because of picture quality or if it's not there.
That said, at that size, it very well could be a flowerhorn as well. I've been looking for a female dempsey to go with my male, and I know the local PetSmart mixes their dempseys with flowerhorns and at that size they can be hard to tell apart. Especially for a big box retail employee who went through a 10-minute training session before being turned out into the world as an "aquatic specialist".
WYite