Thermaltake Aquabay M1
 

Watercoolers often wonder: "How much water do I have in the system?", "Is my flow rate ok?". Thermaltake's Aquabay M1 watercooling reservoir with fill level and flowrate indicator will help you answer this question. It is mounted into two 5.25" drive bays and is compatible with any watercooling system.

Link:
http://www.techpowerup.com/reviews/Thermaltake/M1