Impact of Excessive Rain on Koi Ponds

If your fish are fine after excessive rain (larger amounts of rain in a short period of time or prolonged rain periods), then this is good and you probably don’t have a lot to worry about.

But if your fish are gasping for air, or on the surface a lot, then read on…

The basics: For pH, anything lower than seven is considered acidic, and above seven is alkaline. For healthy and happy koi, between 7.0 and 8.6 is the ideal pH reading.

Normal Alkalinity (Carbonate hardness or KH) KH levels for koi ponds typically range from 75 to 200 mg/L (ppm) CaCO3, with an ideal range of 100-150 ppm. Some test kits measure in dKH (degrees of carbonate hardness) and a dKH range of 5-8 dKH is equivalent to approximately 89-142 ppm. 

The normal pH of rain is 5.0 to 5.5, significantly lower than the level that koi ponds need. 

pH drop and it's mitigation: Excessive rain therefore causes the pH to drop due to dilution of existing pond water. Measure your pond pH and KH using a test kit (not strips). Recommend API Freshwater test kit and their KH/GH test kits. The impact on ponds with low water volume is higher that on ponds with higher water volume. pH fluctuation is mainly impacted due to excessive rain, assuming your pond is protected from run-off water. Rainwater is often acidic, with a lower pH than most koi ponds as you can see from the numbers above. Excessive rain can therefore cause a "pH crash" if the pond's buffering capacity (KH) is low, potentially leading to fish stress or death (seen as fish on the surface or gasping for air). 

Try a water change (like 20%). Replacing a portion of the pond water with fresh, dechlorinated water can dilute the alkalinity and lower the pH. Ensure the replacement water is also tested for pH levels, as it may also affect the pond's overall pH. 

If this doesn't work, make a solution of baking soda (baking soda has a pH of 8.2) in a bucket of pond water, and add this to your pond gradually. Baking Soda is a buffering agent and will help reduce the acidity and bring your pH levels and KH back up. General rule is to add about 1/4cup for 1000gal of pond water. Monitor the pH, KH and ammonia levels, by measuring after some time intervals (say 15mins-30mins to allow proper mixing). Adjust as needed.

pH increase and it's mitigation: While a slightly alkaline environment is tolerated by most koi, a very high pH (typically above 8.5) can be detrimental to fish, plants, and the pond ecosystem as a whole. Particularly in Texas, Limestone is found everywhere, used decoratively and also used to line a ponds. Limestone leaching during excessive rain is a common culprit for high pH, as it can release calcium carbonate into pond water which can increase its pH making it more alkaline.

Measure the

Try a water change (like 20%). Replacing a portion of the pond water with fresh, dechlorinated water can dilute the alkalinity and lower the pH. Ensure the replacement water is also tested for pH levels, as it may also affect the pond's overall pH. 

Using natural pH reducers:

1. Peat moss: Place peat moss in a mesh bag and submerge it in the pond or add it to the filter. It slowly releases tannic acid, lowering the pH naturally.

2. Driftwood: Like peat moss, driftwood releases tannins into the water, contributing to a gradual pH reduction.

Chemical pH reducers (use with caution, as these are very fast acting and cause rapid pH swings, which are bad for koi):

1. Vinegar (acetic acid): Diluted vinegar can also lower pH. Add in small amounts and retest regularly to prevent drastic changes. A common recommendation is to use 1/4 cup of vinegar per 500 gallons of pond water. Please do not dump this in to the pond all at once- add slowly, wait, measure and add more. Also do not add this directly on to a fish as this can cause irritation and

2. Alum (aluminum sulfate): Alum can lower pH and improve water clarity by binding to particles. Follow manufacturer instructions carefully to avoid shocking aquatic life.

3. pH Drop and other chemicals: see instructions prior to using.

Other effects of excessive rain, and their mitigation: If you have run off water mixing in with your pond or have large amounts of debris in your pond- that can cause oxygen level fluctuations that can also be harmful to your fish. The amount of oxygen in rainwater is typically around 10 ppm and it’s easy to assume that rain is not threatening your pond’s oxygen levels (typically dissolved oxygen is a good thing). However, the sudden mixing massive amounts of rainwater with pond water, or run-off water mixing in, potentially causes “stratification” of water (oxygen rich water with a different temperature) than the lesser oxygenated water with a different temperature below in the pond. This variation on the water surface and just below it, causes currents that effectively stir up ("turnover") the water and brings the oxygen-poor water from the bottom of the pond to the surface, leading to fish suffocation. To mitigate this risk, add an aerator. Rain can carry pollutants like pesticides, fertilizers, and other contaminants into the pond, further impacting water quality and potentially poisoning the fish. To mitigate this, build a berm, a wall or a barrier of some kind to protect your pond.