I also see the "slam" construction, as you phrase it. If you wouldn't park a truck on a floor you built, you should get another job
I'm not sure what you're proffesion is, but I know when I build a floor or supervise a floor, the work is not shoty.
Architects and engineers are constantly saying how "shotty" a job is when they have no idea how to do it themselves.
The fact is any 2X6 framed house will hold any tank, anywhere. The only people I would trust as naysayers of that are contractors, and good ones. If you have any concerns, bring a contractor over to take a look.
Will