3 piers were constructed with concrete blocks, concrete lintels and strong cement.
Then, the piers were painted to make them water resistant.
3 box section lintels were put in place.
18mm thick marine plywood was put in place.
25mm thick polystyrene was put in place. 
Black plastic liner was put in place.
New tank was put in place ... 96" x 24" x 30" - 1000 litres.
Here is a slide show that includes all the above pictures ...
Image changes every 5 seconds.
Tank build