The reasons why koi fish lie sideways at the bottom of the tank:

1. The reason why koi fish lies sideways at the bottom of the tank may be due to poor water quality, or there may be something wrong with the filtration system in the fish tank, or Feeding too much food causes water quality to deteriorate.

2. Changes in water temperature may also cause koi to lie sideways on the bottom of the tank, especially when the weather gets cold, because koi are very sensitive to low temperatures.

3. When there is insufficient oxygen in the tank, koi will also lie sideways on the bottom of the tank.

Solutions for koi lying sideways at the bottom of the tank:

1. Change the water of the koi frequently, about once every three days, and clean its excretion objects and food residues.

2. When breeding koi, it is best to control the water temperature at around 28°C and ensure a constant water temperature. When changing the water, the temperature difference between the new water and the old water needs to be controlled between 1 and 2°C.

3. When raising koi, you need to control the stocking density and use an oxygen pump to oxygenate the fish tank. Oxygenation for 1 to 2 hours a day can ensure sufficient oxygen in the tank.