Bichirs grow at different rates even if they are from the same batch. I'm talking samples of at least 10 fish. In my experience, bichirs will grow in spurts when they are above 6". I have had indivduals that have not grown much for about 6 months then suddenly catch up to their clutch mates. If you plan on increasing the temps and the water changes then I would also feed them more often. Try twice a day. They grow much faster on fish fillets than just pellets. I never saw any real increase in growth from feeding prawns as oppose to pellets.