I think it's unlikely that a worker would put the body in a water tank. A worker would want to put the body where it wouldn't be found - at least long enough to move it to another location later. A water tank feeding dozens of apartments would not be a top choice to anyone familiar with the building. Also, it's so exposed up there, with all the high-rises with a perfect view.
I think it's more likely that she went up to the roof willingly, possibly with someone else to take in the sunset, the view, take photos, smoke a cig or a doob. Maybe that's where things went wrong after dark and she was placed in the water tank as a last resort, by someone who thought carrying her back down or out of the building was impossible.
I do think the tank option is weird for someone to hide a body. Unless someone unfamiliar with the water tanks thought that placing a body in water (maybe they thought it got hot?) would destroy DNA evidence or signs of sexual assault etc.
So in my mind, her being found in the water tank points to someone not employed by the building -- or at least not familiar with building maintenance.
It's also someone who is young enough, fun enough, not creepy -- someone who she'd want to hang with up on the roof. A new friend she met or a guy she was flirting with.