Can a hot tub be placed over composite decking?

99%* of all composite decking would not only hold a hot tub just fine, it would be preferred over wood. It is preferred because it will not fail long term due to water damage or termites and cause a collapse or tilt.

As for weight bearing I am not sure there is any calculators out there for composite, however when I have gotten architects involved they basically treat it as "better than PT pine".

The two biggest things to think about when adding a hot tub to a deck is:

1. Joist spacing which you have already thought about. Yes 12" is preferred for heavy load. Adding a 3 joists may be all you need for a hot tub - if you had joistx3 8" apart on each side and dead middle, well that's pretty good against deflection.

2. If planned well you would be trying to place the hot tub above a load point (or multiple). This isn't viable for all decks - smaller decks may only have load points at the perimeter but that also means on smaller decks you should be able to install hot tub on perimeter. Obviously installing something super heavy in the middle of a deck is dumb and will lead to sagging. For what its worth I have seen houses sag after installing 1500 pound safes in the wrong spot (closet in middle of joist span).

"*" the asterisk for the 1% is because I have seen and replaced hollow plastic decking - just in case someone confuses that for composite.

To me, its more a question of the deck structure supporting it. If the posts, beams, joists, etc, are up to it, then your good. The decking itself doesn't have much to do with it, the hot tub spans several joints. The decking is just transfering load from the hot tub to the joints.

Just for good measure, you may want to put some short posts from the slab to the joists under the hot tub.