Oh good lord, do NOT keep ANY ray in a 55 gallon tank.
Just don't. It's too small, doesn't have nearly enough bottom space.
Your LFS guy is smoking something if he thinks an electric ray can be kept for life (or for any reasonable length of time) in a 55 g. Those are even more notorious casualties than bluespots.
I kept my bluespot in a 180g with perfect water, and excellent care, and it STILL died almost a year later.
These animals take special attention and care, and are not easy to keep.