I guess you will have to go bigger and you may need an outside of the pond filter ,koi produce a lot of waste .My koi started out small 7 years later and now they are over 2 feet long and I have 2 large filters and 2 waterfalls .Koi need a lot of oxygen so I also have a 5.8 psi air pump .My pond is over 3 feet deep and one waterfall has a 2 foot section for baby fish to hide .I did not know how big koi got myself and now I know .Thats why all the extra filtering I do .