There is some mis-information in this thread that I'll correct as I go. First off, Velgore, welcome to Tropical Fish Keeping forum.
First on the "red" tetra, the species is Hyphessobrycon eques. Common names vary depending upon store, location, aquarists... as with all common names, they are next to useless. Serpae Tetra is the most "common" of these so I used it in the profile [click on the shaded name for the profile], but Red Minor, Jewel Tetra, Blood characin, Blood tetra, Callistus tetra are others one finds. The scientific names have changed too, as is explained in the profile.
The profile also notes that the temperament of this fish is variable, and the reason is explained there. One thing is certain: this fish is feisty, and prone to fin nip. You almost always see jagged dorsal and caudal fins. Keeping the fish in larger groups of no less than 8 but 12 or more is preferable, and in tanks no less than 30g (with no other fish, larger if other fish are included) can keep the nipping confined. But this is not guaranteed.
Another absolute is that this fish must never be combined in a tank with sedate or long-finned fish; the temptation is simply too great. And before anyone says there fish didn't... just remember we are talking normal
behaviours of a species. The majority of fish in this species will behave according to the above, so the risk is there with the species.
So combining with something as quiet and sedate as a knifefish is out completely. I won't go into the specific requirements to maintain this beautiful fish, but refer you to the profile: Black Ghost Knifefish. It has very specific needs in terms of tank aquascape, lighting, and other fish. And it needs a very large tank while growing. It is inflexible, and this is part of its electrical field mechanism. More in the profile.