That was an acclimation problem, keep an eye on him cause he could still be in trouble. My firefish never lays on the bottom and never did. He was probably in shock. Watch how you acclimate things, this kind of thing COULD result in death. Keep in mind that the longer you drip acclimate something the colder the water gets, so you need to keep it heated during the process, or make sure there are no AC vents aiming near it at the very least.