Along with the species they actually are, growth rate is also influenced by many many other factors. The biggest being diet, water temp, and water quality. Feeding good quality foods and supplementing their diets with vitamins will increase growth. Optimum water temp will increase their metabolisms, allowing them to process nutrients faster and grow faster. Clean water will increase their growth because there will be a lack of contaiminants and growth inhibiting hormones. Doing daily 50% or more water changes will help increase and maintain faster growth.