Um, are you sure the table can support the weight? A 55 gallon tank weighs in at about 500 lbs. with just water, not counting gravel...
If you're certain the legs aren't gonna fall out from under it, then yes, the plywood will work just fine, and you can shim the legs and tabletop to make sure they're level. I would suggest at least 1/2 to 3/4" plywood for this application.
I would make the suggestion that a stand would probably be a safer choice. Something that was built to withstand the weight, rather than a piece of furniture that is not.
Good luck! Hope everything works out.... in case of failing inspiration, I wouldn't see anything wrong in a tank on the floor, but that's just me....(I would live inside the underwater tunnel at Ripley's Aquarium if I could...)