Definitely an awesome pickup. Congrats!
Regarding the integrity of your concrete SOG (slab on grade), if it were my house, I wouldn't worry so much.
In saying this, I have to give a disclaimer. I have a structural civil engineering degree and work in commercial concrete construction, but am not a professional engineer. I work in construction management and haven't done any real structural calcs since college.
This is only my opinion and if you are really concerned, I encourage you to check with a registered PE.
That being said, check out this article:
http://www.structuremag.org/article.aspx?articleID=613
Give it a thorough read. There are tables towards the bottom of the page. For a townhouse basement, you likely have a 4" unreinforced slab ("unreinforced" meaning only wire mesh for crack control and no rebar).
You aren't going to be able to know what the soil bearing was under the slab, so consider the lowest listed value of 50 pci (pressure in psi per inch of soil deformation). For residential, consider your concrete to be 3,000 psi.
So in your case, refer to "Table 1 - 4 inch slab." These tables are used for tabulating allowable steel column loads on the slab in kips (1 kip = 1,000 lb). For the largest distributed load condition - a 16" square baseplate, the slab will support 17 kips. Your tank is much, much larger than a 16" baseplate, so the loading is more distributed.
So yeah, if it were my house, I wouldn't be worried, but it wouldn't hurt to get a second opinion from a PE.
Good luck and certainly keep us posted. :happy0180: