For what you are looking for, try AMEC earth & environmental, they do concrete testing, check the yellow pages to see if there is one in Calgary.
I have used them in the past for concrete testing on foundations. They would probably be your best bet over a structural engineer (they usally have one on staff anyway)
Based on my experience in the industry, older concrete is on average than the current stuff. If you are looking at a standard basement floor, 180lbs/ft2 is not that much, even with no rebar, a garage floor is the same thinkness, and is holding much more per foot.
One thing to watch for is any existing cracking, that might indicate a problem, or it may not, as all concrete cracks, it is normal.
BTW I have over 15yrs experience in the design & building industry, so I do have a bit of an idea on what I am talking about.
You could also increase the loading by addind some sidewalk blocks under the tank, which would spread the load over a greater area.
Hope that helps some.
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