It depends on where your tank is vs the joists. You should be OK if you place the tank directly over a load bearing wall. Otherwise, I would play it safe & either add jack(s) & beams; or if you are going to partition the area underneath, build a load bearing wall under the tank. <That is assuming there is a concrete pad underneath ... as others have said, you can't build a load bearing wall or jacks on top of ground without pouring a foundation.>
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.