Black ghosts get upto 20 inches in size, usually at least 12 inches within a year or so. Capacity of water is not the real issue its the dimensions to make sure they have a good quality of life. A minimum rule that should be considered is that the tank should be 6x the length of the adult size and width 2x the length of the adult fish. Realistically to house them as adults humanely you are looking at a pond in the region of 8-10 feet in length and a minimum of 3 foot front to back. Ideally the same again in height.
They are just not good fish for most keepers. Have you considered other knife fish like African Browns or Glass? Much smaller size 10-12 inches and can be housed in large 6 foot tanks of around 180 gallons.
The Glass Knives are really nice as they are actually gregarious, meaning they like to be with their own kind where as the black ghosts dont like being in close proximity to each other in confined spaces.
Wills