if you are talking about a normal basement slab then the answer is "no problem"
If a tank this small cracks the slab then chances are the slab was not made to meet the building codes and other things have already cracked the heck out of the slab before you anyway.
If there is a doubt?
Then ask over a few of your bigger friends and coworkers...
If you know 4 guys who each weigh about 300 lbs then just line them up and stand back...
If the floor cracks under them and gives way, then you know it's not safe for your goldfish...
