1. Digestive problems: Kissing fish may have problems digesting food, resulting in gastrointestinal discomfort or flatulence. Excessive eating, eating inappropriate food or digestive dysfunction may cause digestive problems, which makes kissing fish feel uncomfortable and tend to lie on their sides.

2. Water quality problems: Water quality problems are one of the common causes of kissing fish lying on their sides. Poor water quality conditions may include high concentrations of ammonia nitrogen, nitrite or nitrate, and unstable pH values. These factors will affect the respiratory system and physiological function of fish, resulting in hypoxia and physical discomfort, which is manifested as the symptoms of lying on one's side.

3. Pressure and environmental problems: Kissing fish are sensitive to changes in water quality, temperature and water flow. Sudden environmental changes, too strong or too weak water flow, and unsuitable breeding environment may bring stress and discomfort to kissing fish. When fish feel stressed, they may behave on their sides.

4. Disease or infection: Kissing fish may be infected by bacteria, parasites or other diseases. These pathogens can affect the physical condition of fish, causing discomfort and abnormal behavior, such as lying on one's side. Common diseases include bacterial infection, fungal infection, parasitic infection and so on.