You really can't raise koi fish with this size, not to mention you don't have a filtering system! Koi fish is growing very fast and needs a lot of room for activities. Your fish tank is too small. It is suggested to add a filter to raise goldfish.
In fact, raising fish is not a simple matter, especially for novice friends. It is even difficult to feed fish, let alone raise it well. If you want to raise fish well, you can't touch the door after diving for a few years.
If the landlord likes fish farming, he can pay attention to our company's latest patented fish tank, which is known as the strongest fish farming artifact in history. This is not bragging, it is based on facts. Look at the size of this water tank Koi fish 1 fish with a height of more than 30 cm, 3 koi fish fish with a height of more than 10 cm and 2 goldfish. These fish have been raised for half a year, and they have only changed water and washed cotton once. Looking at the pile of high fish excrement behind the filter, a sense of accomplishment arises spontaneously.
You should probably understand why water can be kept so well and fish can be kept for so long.