do i need to brace the floor??

You will have absolutely no problem with a 55g. Someone else mentioned 800lbs. That is close, but probably about 100 lbs too high. He added the weight of sand/rock, but it actually displaces water so you can't do a direct addition. The weight of the tank and stand were also high. I think you'd be close to 700 lbs all-in. That's three slightly large people worth of weight. Not a problem for any moderately modern home (and 30 yrs is moderately modern).

Water weighs a lot, but so do everyday items in your home. Consider that a refriderator weighs 500-600 lbs when stocked. Now add a 250 pound person opening the door and you get the picture.

I'd probably put up to a 90G next to a wall on a second story without worrying about it. A 120 might give me pause and cause me to call in someone to look at the house. A 150 or 180 would take some serious consideration, but probably doable with a little reinforcement.

I almost took a job that would have caused me to move and I was planning on putting a 4x4 240g tank in the middle of a room. Now that would have required extra bracing because of the distance to the walls/supports. 3500 lbs or more sitting in the middle of a room in a 4x4 area would not be nice. I was just going to put in an extra column in the basement (and actually run my sump and return down there too! - was going to be awesome... sigh... oh well)

 

Think about everything else you put in a stand, Sumps, pumps, reactors,skimmers , additives, very easy to hit the extra 100# mark. Some rock will actually weight more that the water it displaces. I am sorry I didnt go into more detail but I did hit on the point of the rock and sand displacing the water. It is always better to make sure that everything is calculated in so you dont end up with a disaster. JMPO