Rain can have both positive and negative effects on a koi pond. On one hand, rain helps replenish the water levels in the pond, keeping it healthy and clean. On the other hand, heavy rain can cause pH level to fluctuate.

The optimal pH for koi fish is between 7.0 and 8.5. A pH in this range will provide a comfortable and stable environment for the fish, and will also help to maintain the health of the pond ecosystem.

Rain water usually have a pH of 5.0 to 5.5 which make it very acidic. A sudden influx of rain water will cause the pH on your pond to drop drastically and this will have many negative effects on your fish:

1. STRESS: Low pH can cause stress for the fish, which can weaken their immune system and make them more susceptible to disease.

2. DIFFICULTY BREATHING: Low pH can make it difficult for the fish to extract oxygen from the water, which can lead to reduced activity and stress.

3. STUNTED GROWTH: Low pH can slow down the metabolic rate of the fish, which can lead to stunted growth and other health problems.

4. pH SHOCK: Sudden changes in pH can cause pH shock, which can be lethal for the fish.

To prevent low pH in a koi pond after rain, it is important to regularly monitor the pH and to take appropriate steps to adjust the pH if necessary. This can be done using pH test kits or pH meters, and by using pH adjustment products if necessary. A large water change of at least 25% is recommended after heavy rainfall. Additionally, maintaining good water quality and reducing exposure to pollutants can help to prevent low pH and keep the fish healthy.