Tank weight/placement

[BMW Rider]

I don't think the platform is neccessary. You should be fine with the tank set up directly on the existing floor. The joists system is capable of carrying much greater loading than a tank creates. If you figure your tank will weigh somwhere around 1600-1700 lbs, that seems like a lot, but works out to a load of under 250 pounds per square foot, or about the same as a large adult standing in one spot. Since the tank does not have a habit of jumping and moving around (I hope) it is a static load, which puts much less stress on the floor joists. Given the size of your tank, the weight will all ready be spread over at least three joists.

Most modern constructuion methods use adhesives to help attach the subfloor to the joists. This basically causes the the entire floor to become one large structural component. It can actually suffer a significant failure in an area, and still remain strong enough to support major loading since the load is spread out over a larger area than it would actually appear. This is the reason that engineers are able to create large spans with seemingly flimsy materials.