Drain valves are made for the 4.0 Quartz Fountains. 

They come in 2 sizes. 14mm and 12mm

The T4 Fountain uses a 14mm Drain Valve

The T6 Fountain uses a 14mm and 12mm Drain Valve

Because they are Borosilicate, DO NOT heat them inside of the quartz, or they could expand and get stuck.

*quartz not included