Effects of Rain in the Pond

Rain in the Central Valley has been rare in recent years, but we welcome it with open arms. Ever wonder if rain can benefit or hurt our koi and pond environments? Some might think the rain will benefit the pond because it eliminated the need to add or change the water since the pond is getting fresh, clean water from above. However, there are many variables to consider. Think about the air quality in your area and what the rain might wash from the sky. Is your pond above or below ground level? Is there any structure, like a bridge or pergola that is treated with toxic stain that could wash off into the pond?

A below-ground level pond could get debris washed into the pond water or pesticides and chemicals present in your landscape that could wash into the water and be toxic to your koi. Dirt washed into the pond would cause the water to become muddy and unclear and could carry sticks and leaves that cause blockage inside the filtration system.

“The quality of rainwater can vary from area to area, for instance, it is felt that heavily industrialized areas are more likely to receive polluted rain than lightly industrialized areas. We need to throw into that discussion arena the effects of major motorways (traffic fumes), airports (airplane fuel emissions), petrol-chemical plants and farming.” – Quoted from PondTrade Magazine article.

Is the pond getting runoff from your roof or other potentially dangerous materials? Some roofing material includes zinc, cadmium, lead, or even copper, that is toxic to koi.  

“There are two inter-related factors to consider before deciding to use rainwater for fish:

**pH** values affect fish health considerably. The typical pH of rainwater is about 5 – 6. However, industrially produced sulphuric oxides and carbon monoxide in the atmosphere can reduce the rain’s pH to 4 or below. Whereas such a low pH could be beneficial to fish such as breeding discus; it could be extremely harmful to fish needing alkaline waters such as koi which require a pH of 7+.

**Alkalinity** is missing from rainwater. It is required in various fish waters to buffer and stabilize pH, some fish require far harder water.” – Quoted from PondTrade Magazine article.

So, what can we do to prevent a pH crash is to measure your pH before the rain.  Test your pond water before the rain starts to get your baseline pH. Also, test your tap water to get that pH level so you know what you are filling your pond with.  If you have never tested the pH of your water, you might know what is normal and what is not.  Below is a chart of water quality ranges recommended for koi ponds.

Koi Water Quality Ranges*

Ammonia≤ 0.1 mg/L (ppm)
Nitrite0 mg/L
Nitrate< 20 mg/L
kH (alkalinity)> 100 mg/L
gH (hardness)> 100 mg/L
Temperature32-85F (0-29C)
*Chart courtesy of Dr. Jessie Sanders, cafishvet.com

So what steps should you take to avoid a pH crash?

Test your water parameter of pH and kH, depending on how fast the rainwater is running and how much you get. If you get a lot of rain, test every hour and log on a sheet the date/time/parameter measured.

If the kH is starting to drop BUT the pH is staying consistent, then add some buffering agents to bring the kH back up, such as calcium carbonate or other products available through koi pond dealers or local pet/aquarium shop.  If both the pH and Kh are dropping, then you will need to add additional buffers to prevent a pH crash such as baking soda, which is popular amongst pond owners. You can keep a big bag and store in your garage for emergency use.

“Example: If you had a 3,000-gallon pond with an alkalinity level of 30 ppm and you wanted to raise the Alkalinity to 100 ppm, you would need to increase the level by 70 ppm. To do this, you would add 49 oz of baking soda to the pond, this would reach the desired level of 100 ppm.” Courtesy of Sacramento Koi

Ideally, with a koi pond in rain, you should not see any change in your pH more than 0.5. Your kH should remain above 100 mg/L, dropping to 50 mg/L at the very least.

Do your research before adding anything to your koi pond. Be cautious and carefully vet products available online. Talk to other koi hobbyists or CCKS members to get their recommendations. Also, consider performing a water change as it may benefit the koi. Do not forget to add your water conditioners, such as de-chlor, if you have city water to prevent a high chlorine tragedy.

Purchase a water test kit, if you do not own one. I recommend liquid test kits, such as API, instead of test sticks. When purchasing the test kit, make sure it contains the wide-range pH option. You will need to purchase the kH test kit separately as I have not found a set that contains both.

Aquascape Test Kit
Aquascape Brand Test Kit
Pond Master brand water test kit
Pond Master Test Kit

Most test kits can be purchased online. Do not forget to write the date you purchased the item on the test kit box to keep track and ensure it does not expire on you. A simple notebook to track the rain/water test is also recommended.

So, enjoy the rain while it is here in the Central Valley! Hopefully, this information helps you care for your koi and pond water during that time.