180G, 1800 lbs plus sump, rock, stand, sand... you're well over 2000 lbs. spanning over 5 joists you're at 400 lbs per joist. You may be spreading the weight over more joists via the sub floor. So that should hold (I'm not a structural engineer mind you), but if you have access to the floor, it wouldn't hurt to reinforce the joists.
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