There is a good chance that your bichir will swallow some gravel. Bichirs, especially young ones, are very clumsy feeders and have poor eyesight, sand is best for bichirs but cery small gravel, say 2mm stones max will probably be ok.
if you love your bichir you will change ur substrate immeadiatly...Mine choked on a piece of gravel... so i set up a new tank with very large gravel that seems to be doing great...my new bichir is doing very well and is very happy now...