I caught it in a reservoir in Japan. I haven't seen anyone in China who knows what fish, fish and freshwater fish are.

According to the information you provided, it is difficult to determine the exact species of this fish. However, according to the appearance of the fish and the fact that it was caught in the reservoir, this fish is probably some kind of freshwater fish. Here are some freshwater fish species that may be similar to this:

1. Carp: Carp is a widely distributed freshwater fish, distributed in Japan, China and other Asian countries. They usually have large scales and mouths, and obvious beards.

2. Crucian carp: Crucian carp is also a widely distributed freshwater fish, mainly distributed in China and other parts of Asia. They usually have smaller scales and mouths, and short beards.

3. Snakehead: Snakehead is a large freshwater fish, mainly distributed in South Asia and Southeast Asia. They usually have black or gray scales and mouths, and obvious beards.

4. Perch: Perch is a widely distributed freshwater fish, mainly distributed in China and Japan. They usually have large scales and mouths, and obvious beards.

It should be noted that because there are many kinds of freshwater fish in different regions, the same fish may have different names and appearance characteristics in different regions. Therefore, if you want to further confirm the species of this fish, I suggest you consult a local fish expert or refer to the relevant fish identification guide.