A tank is a storage system for liquids. It is capable of storing up to 16000mB (16 buckets) of Oil, Fuel, Water , Lava , Mob Essence , Sewage or Sludge per block. You may use BuildCraft Waterproof Pipes to pump into them. You may pump out of them with any engine and a wooden waterproof pipe. They are most useful for keeping resources from the Pump . You can also use a bucket to input or output liquids from the pump by right clicking. Similarly, a Deployer can be used to output liquids into buckets.
If there is very little liquid in the tank, you may see a small amount of liquid flicker noticeably.