A ballast tank is a tank used for ballast (weight carried to correct trim, heel, draught or aid stability). Generally, they carry a quantity of seawater, the level of which is controlled to match the condition of voyage. Sometimes, the ballast tanks will be filled with freshwater, which is less corrosive. The level of these tanks tends to be held more constant.
The location of ballast tanks will vary with their intended purpose. To aid in stability, the ballast tanks will be placed low and divided from port to starboard (to reduce free surface). For tanks to correct trim, they will be placed at the forepeak or aftpeak spaces. To correct heel, they will be placed at maximum beam as wing tanks.
Ballast tanks filled in one geographical area, and emptied in another, can lead to the transplantation of non-native species. A number of methods are used to reduce or prevent this; such as continuously or periodically cycling ballast water en route or the use of biocides to treat ballast water.