Saltwater doesn't weigh 12lbs/g. It does weigh a little more than freshwater, but just marginally. If the tank truly holds 1000 gallons, I'd estimate it (just the water) to weigh just under 8000lbs with the displacement from the sand and rock. That's 3400lbs less.
I'd second the recommndation to hire a structural engineer though. You're definitely going to have to reinforce with some steel beams, and probably lose the area directly under the tank down in the basement for supports.
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