The reasons why guppies shrink their tails:

1. The water temperature changes too much. They are relatively sensitive to changes in the outside world. If the water temperature in which they live suddenly changes significantly, they will be stimulated, causing them to shrink their tails and fins.

2. The water quality is too poor. Although they can live in poor water, if the water quality is too poor, bacteria can easily breed, causing them to shrink their tails and fins.

3. Bacterial infection. If there are bacteria in the food they are fed or if they are not treated in time after being injured, it will cause their tails and fins to shrink.

Methods for treating guppy tail shrinking:

1. Keep the water temperature stable. You can first keep the water temperature at around 26°C, and then use a heating rod to keep the water temperature in this range. When they adapt to this temperature, the shrinking of the tail and fins will be alleviated.

2. Change the water in time. The water needs to be changed immediately to keep the water clean. Open the filter and clean the filter material to prevent bacteria from affecting their bodies again.

3. Clean and disinfect. It is necessary to first use potassium permanganate solution to clean the sick fish, and then use potassium permanganate to disinfect the fish tank, which can avoid the growth of bacteria and reduce the chance of infection.