Hi !!! I'm a complete novice who has just built my pond and was wondering what is a good media to have to keep koi ?? I built my pond 2 yrs ago and incorporated a back filter box to also act as a water fall. My pond is 6ft x 6ft x 40 inches deep sloping on the bottom from 32 inches to 40. It is roughly 897 gallons ,4078 litres. At the back is a tank which I made a waterfall in and also planned on using it as a filter box ! I've got all different types of sponges in there and different plastic media but I've seen this rock media called Alfa grog and was just wondering if it would be any good for koi ???? My water is getting much cleaner now as my old pump was enough for my water volume so I brought a much better pump. I brought a baby koi 2 weeks ago but he lasted 3 days !!! Any suggestions on media and knowledge on koi would be very helpfull !!!