Another one that I've heard of happening before is that there was so much movement the bichirs became very stressed and in extreme cases died. I look at your stock and see (other than polypterus) a couple of SD and clown loaches. I've never owned a clown loach, but from I've read and seen they like to move a lot. Especially in a small size tank all the fish could have just been very stressed from movement.
They do love to move a lot ..I didn't though about this one
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