48"x12"x30" is roughly equivalent to 75 gallons. 1 cubic foot = 7.48 US gallons. It's not a"standard" tank by any means, so I definitely would not recommend some fish that work in a standard 75/90gal, like any tang.
That said, it would make an awesome seahorse tank.
You provided measurements in inches:
Width: 48" (121.92cm)
Height: 30" (76.2cm)
Depth: 12" (30.48cm)
Your tank's volume is approximately 17,280.0 cubic inches or 74.8 U.S. gallons, which is approximately 291.0 liters.
wolf eels are crazy! if you like agressive that is perfect. i was stupid a bought one on a whim without research and had him until i saw my six line wrasse hanging out of his mouth. lol never had a undy though. i've read they're pretty agressive also though.