I wouldn't trust any websites with Gymnotus information. Somehow, the common name "zebra knifefish" got attached to the scientific name Gymnotus pedanopterus. This was a mistake that somehow has really stuck.
The true zebra knife is Gymnotus tigre (see photo) and it has the most distinct color pattern of any Gymnotus.
Gymnotus pedanopterus really doesn't look very distinct and as far as I know it has never been exported as an aquarium fish. Unfortunately, there isn't really a great resource for telling all of the gymnotus apart. Many of them are separated by very subtle differences in head shape or bones.