Bifferwine
I am a girl
48" for a 55 is the standard 55 gallon tank size, not a long, so there are lots of options for lights. You have a choice between metal halide, T5 or LED. Do you know which you are leaning towards?
I would say two of these. My tank is 48". Two gives perfect coverage.
I'm still on the fence about recommending these to anyone. It far too soon to judge the quality of light and the longevity. Yes, I'm happy at this point but I have only seen minor, if any growth. I have softies mostly, three SPS frags and one LPS. Everything looks great but only one Zoa is showing any growth. To be fair I do have other issues I'm working on.