I am installing/rebuilding an older model 6-man octogenal jacuzzi outside on an existing deck. The current deck there is made of 2x4's on 18inch centers with 2x6's running perpendicular to the 2x4 base. It just sits directly on the ground, and I'm wondering if this is insufficient for the tub. I'm pretty sure that I'll need to put another set of rows of 2x4s between the 18-inch-spaced ones, but other than that should it be alright? Thanks for the info ~

If the 2x4 joists are directly on the ground, all the way, you should be just fine as it is. If they are suspended any distance above the ground, I would rebuild the whole thing with the proper dimention lumber.