The inch per gallon "rule" is just a very basic guideline for those who are new to the hobby to prevent them from overstocking their tank while they are still going through the learning process. Once the new fish keeper has a bit of knowledge and experience under their belt the inch per gallon rule can be thrown out and stocking limits can be pushed as high as 3" per gallon depending on fish sizes, tank size, maintance, filtration and airation but during the first months/years before this knowledge is aquired and before the tank is matured the inch per gallon rule saves a lot of fishes lives and stress for the fish keeper.