Only just seen the tag, looks like it's been covered already, but will add for future reference.
Like a lot of young animals, producing growth hormones identifies them as juveniles to mature animals. Adult animals will kill or eat the juveniles to prevent them from becoming competition. Bichirs have sensory pits and tubular nares which detects the slightest hormone changes in water, and the juveniles produce them up to 7 inches (+/- 10%). Any juvenile bichir this size or smaller is at huge risk of being eaten. Also, bichirs (especially lower-jaws) under 7 inches kept with others of similar sizes may try to each other too; again eradicating competition at the earliest opportunity.
As Hao showed though, provided they are above 7 inch, you can have huge size differences between your bichirs. 8 inches with 24 inches isn't unheard of.
Although try to remember, bichirs are opportunistic predators which may still eat each other, it's just far less likely to happen with bichir mixes over 7 inches.