Lionfish are prdators, they do not expend their energy swimming to and fro, like a tang or angel or wrasse, etc...They conserve their energy by hanging in one place for long periods waiting for prey to swim by, then......BAM they strike. If fed live feeders (not recommended) you will see their hunting posture. The lions biggest impact on the tank will be on the bioload, they are messy eaters and produce alot of waste. I seem to remember that carnivores seem to produce alot of undigested/partially digested waste, so it's basically like throwing part of the shrimp you fed it, into the tank to rot. The other impact is the one on the small fish and shrimp in your tank.