DO read the article, a small knife will work in a 55 gal till it gets big, mine is still small enough to be in my 55, however you have to make plans for the future, I will be building a 1100 gal tank for all my fish, because of the ghost knife, not because it needs that much room, but the fact that I want mine to exibit normal behaviour, all you will get with a 9 inch ghost in a 55 gal is one unhappy fish, HYPERASHEL, Ghost knives are blind but mine does not bump into everything, if yours is then you have too many fish in one spot, too much light and no floating plants, Please research this fish again.