The size will tell you in the future. There are many naked back knifefish out there that looks very similar in patterns but most grow to max of 10-13 inches. If it grows bigger than 13" then there is a chance that it could be a g. carapo or g. tigre. If it grows bigger tha 16" then it's a g. carapo. The patterns looks like a g. carapo, but I'm not 100% as I'm also getting started on keeping this fish.
I'm pretty sure this is Gymnotus carapo because the head is a fairly consistent brown color. Gymnotus tigre has pale-yellow spots on the head. Also the stripes on the tigre are very shart with high contrast.