We just had a "planter" built and are wondering if we can safely change
it over to a "pond" with some minimal changes? It is built level at the
top, but the ground is on a slope of about 1 ft. per 15 ft. so the base
bricks don't form an entire solid line around the perimeter until a
couple of bricks up from the lowest point.

The planter is constructed with the 9 inch wide/ 4 inch tall landscape
blocks. We have the bricks glued on top/bottom with a "liquid nail"
type adhesive. The bottom rows of bricks is either buried in the clay
earth or else half buried. Interior dimension is 13.5 ft. in length x
4.5 ft. wide. Depth is 2 ft. at the bottom of the slope and about 18
inches at the other end.

I figure it will hold maybe a bit over 600 gallons in its present
form. Should we dig it deeper in the very center or line the sides with
cement to reinforce it? Did it need to have a reinforced concrete base
as opposed to building on clay?

My original idea was to fill this with soil/plants/fountain; but the
pond sounds like more fun, albeit more work.
We are in zone 7b, so weather is not too harsh.