In 1.0.0 gamma, when you craft a tank, it doesn't have a tooltip, and it stacks just fine. When placed in the world and picked up again, it now has a tooltip saying 0% and will not stack with other empty tanks.

I've made some fixes to this, they should now stack a lot better. If this continues to happen in the odd occasion I'll do what I can to track it down. What I can recommend is to place the tanks that wont stack, put them together and then break them again. They should stack. Originally the tanks were never identical as far as NBT data, but now they should match up provided they contain the same amount of liquid.

Test it and get back to me, and thanks for testing out OpenBlocks :D