A few threads about knifefish recently indicate that many consider these fish without realizing the requirements. This article explains these. This is not a fish for the inexperienced aquarist, nor will it be healthy in too small (under 10 feet--yes, he says 10 feet) tanks.
Predators: The knifefishes | Features | Practical Fishkeeping
Predators: The knifefishes | Features | Practical Fishkeeping