Sand because the way a bichir feeds they can ingest sand with little or no consequence the same can not be said about gravel which can cause intestinal blockage and death if they can't cough it out-Anne
I just set-up my bichir tank with sand after everything I have heard, I just saw a bichir with a pebble in his mouth at an lfs the other day, that really had me worried! I'll use sand just to be on the safe side!