Easy enough, work out the volume (2m x 1.5m x 1.6m) and multiply by 1000Kg (one cubic meter of water is equal to one metric ton at 4degC which is the temp at which water is densist)
So your answer is 4800Kg or 4.8 metric tons.....so a lot
Thanks Mark, the question was not for me, but just got me thinking as well, our fish tank must weigh a couple of Tonnes, no wonder the unit it is beginning to bow...

Whats the floor loading like!

It's the loft that worries me....

1930's house so decent material used... 
I would't kid yourself to much on that..... 
They didn't use floor joists in the loft even then.....and many of the joists used back then would not meet the requirments of C16 or C24 timber for todays standards....and hence why you see many split beams in old roofs giving roof sag 
Not sure what that means but iirc, they are fully covered, I think they are 8 inches deep and 3 or 4 wide, don't quote me on the width though.
I seem to remember that they were OK for a loft conversion, (10 years ago)which we did not do..

Thats the thing, size means nothing (and building control would know this....an architect or builder possibly not sadly)
The specs describe the maximum allowed number of faults (for example, natural features such as knots, wane and slope of grain,
plus splits and shakes which may have developed as
a result of drying).
Plus joist spacing is very important as well as span.
E.g. In old money a 9inch by 3 inch joist to the above specs will only span 5m (ish) at 400mm centres.