The tank does not way any more then your fridge does. If your fridge has not blown through the floor, the tank won't. 55 gal tanks are not that heavy in regards to pounds per square inch.
Builders know in advance where a stove or refrigerator or a bathtub goes in townhouse layouts and they reinforce the joists specifically for the additional weight, so I'm not sure that's a good comparison in general. Anyone know what the pounds per square inch requirements are in the building codes?