look anything like this?



http://www.accessdiscounts.us/sku-26125.html

If so, its likely a magnetic ballast. It should fire a 400w metal halide bulb. Does it have a mogul socket attached to it? Or is it just 2 wires coming out? Photos of what you got would help too

As for the aquarium end of the deal, it may not be the most efficient or pretty ballast, but if it fires a bulb, it'll work. Most of the nicer ones in the hobby are electronic ballasts (more efficient than magnetic ones) and come in cases to disperse heat. A 55g tank is pretty long (4ft) so a single 400 would not cover the whole distance. Plus it may be too much light for a tank that size. A 250w or (2) 175w would work perfect for it! I did a single 250w metal halide on my old 55g and it worked out well. Some of the sides were a little darker, but it worked well for low light corals.