Hi Wee Yang,
I have some suggestions for your customers , when building a fish tank , always think of ways to make water changing easy. One way is to incorporate water inlet and outlet into the sump tank below the fish tank.
Water inlet can be direct from the tap controlled by a float valve like those used in the toilet. Water outlet can be UPVC pipe or hose leading to a drainage point.
Also encourage them to have a sump tank as big as possible , if not same size as the fish tank above.
William