The reason why fish don't open their mouths is probably caused by low temperature. As a temperature-changing animal, the activities of fish are greatly affected by temperature. When the temperature is high, they love to eat, but when the temperature is low, they will not open their mouths. In view of this situation, it is necessary to raise the water temperature properly, but don't raise it too high or too fast. Too large temperature difference will also be unfavorable to fish.

Although many fish are omnivores, they also have something they don't eat. If the food is not good, they will not open their mouths. In this case, the best way is to change it to eat, and it is best to feed it with live bait, which is the food that easily arouses its appetite. If you are fishing, you can use some bait with fishy smell.

If the water quality is poor, the fish will not open their mouths very much. It may be that the ammonia nitrogen content in the water body is too high, or the water quality is turbid. If you raise fish at home, you must pay attention to changing water, or prepare a good filtration equipment, and always feed water for filtration, which can also maintain the water quality so that it will not deteriorate rapidly.

If fish are sick, they will not open their mouths. In this case, there are usually other phenomena, such as rubbing the cylinder, shaking the fins, or having some abnormalities on the body surface, such as white vesicles or erythema, etc. After discovering that the fish is sick, it is necessary to stop feeding, treat it first, and then feed it after being cured.