I've had two very nervous sharks for some time and they have grown nicely. Walk by the tank and one or both will dart, often hitting the lid. Worried they will hurt themselves....often see nicks after they try to jump out of tank through the lid. Have lowered temp to 74 in the 55 gallon tank with no desired calming. From what I've read....doesn't seem to be normal behavior although local fish store says that they often find sharks on floor.