When I had my bichirs one of them was always at the top and the other at the bottom. When I first got them they both stayed at the top then one went to the bottom.
I think its quite common for young birchir to stay at top of the tank....
as my 3 baby birchir also always like to swim at the top...they are around 3 to 4 inches....
i think it is ok.. during this period when u jus release them into the tank, they will hover ard the top of the tank and get quite jumpy. do rem to cover ur tank and switch off the lights. once they are acustom to the tank environment, they will start to slack ard at the bottom of the tank.