The idea is to spread the weight of the tank over several joists. Sistering would help, but I would place the tank over a base spanning several joists. You can do this by adding a wider base under the tank, or getting between the joists with some blocking, so the joists work together. Add blocking or cross-bridging between 4-5 adjacent joists, securing it with both glue and screws. Or use Strong-tie bracing. If you can remove the plywood above the joists, you could also glue it before nailing it. That helps with the spreading of the loads...
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