What the whole thing weighs once it is running, and what that works out to per square foot of floor.
Seawater weighs about 8.55 lb per US gallon, more than fresh water at 8.34. The total here is what sits on the floor while the tank is running, which is the number that matters for a stand and for a joist.
total = water 304 + glass 70 + sand 27 + rock 27 + stand 68 kg = 497 kg
Seawater at 35 ppt and 25 °C has a density of 1.0233 kg per litre, which is 8.55 lb per US gallon against 8.34 for fresh water. The two percent difference is small per gallon and adds up across a large system.
Glass weight assumes soda lime glass at 2.5 g/cm³ across five panels, four sides and a bottom. Rimmed tanks and braced tops add a little. Low iron starphire glass is essentially the same density, so the number holds either way.
Floor load is total weight divided by the footprint, and it is the figure that decides whether a tank can go where you want it. Residential floors in the US are commonly designed for 40 lb per square foot of live load plus dead load, and most tanks over about 75 gallons exceed that locally.
Concentrated load spread across a stand is not the same as a person standing in one spot, which is why plenty of large tanks sit on wood floors without incident. It is still worth checking rather than assuming.
The practical guidance: put a tank against a load bearing wall, running perpendicular to the joists so the weight spreads across several rather than sitting on one. Ground floor or a slab removes the question entirely.
Above the ground floor, and above roughly 120 gallons, get someone who does this for a living to look at it. A failed floor costs far more than the consultation.
Sources: Aqueon tank dimensions (PDF) · UNESCO EOS-80 / Millero & Poisson 1981