if you're lucky, the roof joists will be running perpendicular to the orientation of the tank, so that you can bolt a unistrut across several joists.
if not, you'll have to run intermediate members and THEN the unistrut to support the light.
none of that is complicated, but depending on what's up above, it could be difficult to access. if you don't have individual AHU for each unit, there's a good chance you have ducts feeding other tenant spaces running through your space. you won't know til you look up there.
one word of caution though, check your lease agreement. these modifications might be NBD if you own the place, but if you're leasing the space, making structural modifications (drilling into the roof joists COULD constitute structural modification) may be a breach of contract.