A standard 55 gallon tank is 48 inches long. That sounds about right for a 4-bulb T5 fixture. They also have 8-bulb fixtures that pack twice as much punch. Those are better than 4-bulbs, but go with what you can afford. I am running the 4-bulb Nova Extremes on my 55 gallon tank, and I've been able to keep anemones and SPS just fine.