Yes they do ok with knives as long as the sizes are right (one can't be eaten by another), knives should do fine w/ ample cover, so heavy planting would be a good thing and you won't have to put PVC in. Bichirs love PVC and they'll most likely fight w/ the knife over it or each other, unless there's enough of it for all of em n then you just won't ever see your fish haha
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