a empty 75g tank is aprox 100lbs
my 90g tank empty is around 125lbs
in the 75g tank I estimate about 50/75lbs of sand depending how deep a sand bed you like.
live rock is a lb per gal
now water 75gX7g=525lbs
total 775lbs is your approx weight minus the stands weight so that would need to be added to the total for a final tally
my home was built in the 1950's and the floors are rated approx 1400lbs per sq foot
I keep my 90g and 40g along with both there stands and sumps on the same floor on the same wall . for a total of 1740lbs with no issue ,