10 Koi Continue to Thrive Over 2 years in 50 Gallon Tank

I have been caring for 10 koi in a 50 gallon tank for over two years.  Except for their growth stopping , they remain, healthy, active , attentive and almost always eager to eat the amount of food they can consume in 30 seconds.  I use undergravel filtration, with a 2 inch gravel bed and monthly 80 to 90% water changes. Experts have said that they will out grow the tank, but so far I see no sign of it.