Can I put plastic film on the bathtub to raise fish? Is it harmful to koi fish? thank you

No problem at all. As long as your plastic film doesn't break, you can buy a slightly thicker one, but it's really impossible to have more layers!

Otherwise, you can directly use the bathtub to raise fish. When you can move the fish outdoors next year, you can clean and disinfect your bathtub. You can choose potassium permanganate and 84 disinfectant to disinfect alternately once each time, and there will be no problem ~

In addition, your bathtub had better not be the one with a slope, otherwise koi fish is likely to run out of the bathtub along the slope, and you will be heartbroken!