Sorry it took me so long to respond! There are probably a million "banded" knifefish in the knife family. But I'll take a guess that you're either referring to Gymnotus pedonopterus or carapo.
1. Both of those would be too large for a 48x18 (75g)
2. Both of these are very aggressive fish and would need some hardy tank mates
3. Where are you looking at buying your specimens?
Mez