Do Mono Sebae's prefer to be by themself or with other Sebae's?

I am starting up a 55 gallon brackish tank. At the pet shop there were two other Sebae's in the tank mine was in, but they were chasing eachother aggressively. Wasn't sure if they would just stress one another, so I only got one, but it seems bored staying primarily in same place except when feeding. Any advise?