Are you sure you have it built right.
The entire frame must be made of Tank Walls. No values or glass can be used.
(ex. on each 5x5 side you look at, the most outside blocks must be made of Tank walls).
X X X X X ----- X= Tank Wall
X A A A X ----- A= Tank Wall, Glass or Valve
X A A A X
X A A A X
X X X X X
Also make sure no blocks or things are inside the finished tank. This also includes dropped items too. Also any wraith lamps in the area, destroy them as they sometimes cause light blocks which can stop a tank.