Lionfish are pretty hardy. What size tank do you keep him in? Did you Q him at all? He is probably fine, just adjusting. They are known to display some pretty weird and confusing behaviors. They like low lighting though, and need some overhangs, or caves to rest in. Some have even suggested that high lighting can over time damage a Lionfish's eyes. Do you have him in a reef tank? If so, this may pose a problem --- reason being, is though in the wild, this is more their natural habitat, in a tank since they have a big appetite, and a LOT of waste, it can be hard to keep up excellent water quality, and they may "snack" on some of their tank mates. You can feed him fish and other crustaceans such as silversides, shrimp, uncooked prawns, krill, and even bits of fish fillets of the marine origion. Don't house them in a tank with small crustaceans and fish (that you want alive), because even though they generally are not too agressive or territorial, they can, and if hungry, will eat things smaller than themselfs.
Some of the wierd behaviors you may encounter (but are all normal) are...
- Lying upside down against a rock or overhang
- Flashing its fin spines up and down periodically
- Virtually sitting stone still
From time to time, your lionfish may cough and shudder. They do this when they shed their outer layer of skin. It looks like a fine mucous peel. They do this to rid themself of parasites and algae, which tend to attatch to their skin. If he does this a lot though, it may mean that your water quality, is too low for his liking, causing irritation.
If he is eating fine (usually they do best fed early in the morning or late at night, as this mimics their natural hunting times), and does not seem to have any physical signs of irritation or discomfort, I wouldn't worry too much for now. Just keep your eye on him, watch your water, and other tank mates, and I am sure he will be right at home, and ready to go in a few days or so.
BTW, yes that other tank was way to small for your guy. It could play a role in his current condition, but since I assume yours is much bigger (tank), there is no reason he shouldn't adapt/recover quickly.
P.S -- How did you recieve this fish (i.e -- in a bucket, a bag?) reason I ask, is that netting these fish can be really hard, and cause damage to the spines, as they are very fragile. If he was bagged, check out his spines for any potential damage.
--EMI