A dual water-tank which is composed of two parts; The left part contains recycled water and the right part contains tap water.

Outwardly, it looks a normal water-tank except two levers and a single LED.

So, when the left part is full of recycled
water, the LED becomes green light, then
we can pull the lever for recycled water
to flush the toilet.

Inversely, the LED stays on red light,
indicating not full it yet, then we can
pull the lever for tap water to flush
the toilet.