Is your iron tank correctly forming the multiblock? In other words, do you get a GUI if you rightclick it?
If not, you probably made a mistake in building the tank, such as:
- Choosing an invalid size - only 3x3, 5x5, 7x7 and 9x9 are allowed, with a height of minimum 4 and maximum 8 blocks
- Placing valves or gauges on the frame (the edges) of the tank - only the basic walls are allowed there
- Not leaving the insides empty - you only need to build the outer hull, the middle part needs to be empty, much like a coke oven