Hi Marc
I would agree with Baz that your existing concrete slab should be fine - but it would probably pay to lay some good quality evenly spaced sleepers (about 75mm depth) on your concrete slab as CS suggests before placement of your tank on top - too easy. If you worried about rubbish blowing in between/under the sleepers (or perhaps for a cleaner look) the alternative would be to use the sleepers to make an open box that sits on top of your slab then fill it with compacted quarry rubble before placement of your tank on top.
Dennis, like your set-up. From one city slicker to another - I have 4 huge significant river reds in my front yard and 1 in the back, all of which dump so much rubbish on my roof (not to mention the two families of possums we have), I simply can no longer use my water tank. Short of removing these trees, I don't need a sales person to tell me I need to take some preventative measures similar to yours - depends on the potential contaminates in your local area, no matter where you live
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Clear skies and tanks to all
Cheers