Rain Water, Benefit or Hazard for Ponds?

Published on October 1, 2010

rain_water_umbrellaWhat does rain water mean for fish keepers? Is it a benefit or a hazard? That depends upon how you look at it.

  • Rain water can be a benefit by:
    • Providing a free water supply, particularly beneficial if tap water is metered and charged accordingly by the water supply company.
    • Being free of chlorine and chloramine.
    • Being Soft.

The above contents could send fish keepers out on a fast buying spree to get large rain water barrels so they can start collecting rain water for their fish as fast as possible. But before doing so, think carefully about the hazards. ‘Free’ is a word we all like but on the basis that ‘there is no such thing as a free lunch’ – could there be a price to pay or, more importantly, a price for your fish to pay? Let’s explore rain water to find out.

It is incorrect to suppose that rain water is just pure water (H2O). As water is evaporated from the earth and drawn up to form rain clouds, other dissolved substances may also be taken up with the water. Research has shown that falling rain can dissolve, carry and be impaired by, gaseous pollutants in the atmosphere, which were created by industrial and human activities. (“Industrial Contamination,” Seip and Toll 1985; “Degradation by Human Activities.” Ayoade 1988 & Pickering and Owen 1994.)

 The Nigerian Government set a 2008 deadline to end all gas flaring by oil companies after research showed, among other things, that lead levels were above the maximum allowable in potable water by the WHO (World Health Organization) and higher values of lead were found in the rain water than the ground water.

The quality of rain water 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.

It has been proposed that particles can also be carried in rain clouds; this is supported by:

(A) Dependent on the wind direction, some heavy rainfalls around Brentwood (England, UK) deposit sand. Anecdotally ‘the sand’ had been carried from the Saharan Desert in North Africa across Europe to Eastern England.

(B) The Chernobyl nuclear reactor accident in Russia was seen by the UK government as being directly responsible for polluting some of the UK’s Welsh Hills – for some years governmental restrictions were placed on allowing lambs produced in these areas to enter the food chain.

Research undertaken at the University of North Carolina (USA) also showed that the components of rain water at a coastal site in North Carolina varied according to the time of day or night samples were collected (G. Brooks Avery, Jr; Joan D. Willey; Robert J. Kieber, 2000).

If you live in an area, which has very little industrial activity and the prevailing wind does not come from industrialized areas then the rain water may be perfectly acceptable for fish use. The U.S. EPA recommends that in some situations, rain water collected for drinking may need further treatment to disinfect the water or to filter out other impurities from the rain water supply. This water would probably not need disinfecting for fish but having experienced tremendous damage done to koi by heavily polluted rain fall,**it is a fallacy to think that rain water is always fish safe.** The main contaminants can be roughly grouped into organic, inorganic, radioactive and acid/base.

Substances in collecting and storage materials can also contaminate rain. For instance, roofing materials: Experiments in Paris showed the leaching of zinc, cadmium and lead in wet weather (Chebbo et al., 2001). Additional research has shown that zinc dissolved from roofing materials is mainly in the form which is damaging to both fish and aquatic plants (Heijerick et al., 2002). For human safety, The NSF Rain Water Catching System Testing Programme reviews products used to catch and store rain water; further information should be sought from manufacturers re fish safety.

There are two inter-related factors to consider before deciding to use rain water for fish:

**pH** values affect fish health considerably. The typical pH of rain water 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 rain water. It is required in various fish waters to buffer and stabilize pH, some fish require far harder water.


For some fish species and in some areas, dependent upon how the rain water is collected and stored, rain water can be very useful to fish keepers. However for other fish species and/or in contaminated rain areas it may be advisable not only to avoid using rain water but also to erect covers over outside ponds to prevent the rain water from entering the fish water.

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9 thoughts on “Rain Water, Benefit or Hazard for Ponds?”

  1. We have a small pond approx. 2300 liters, it is only 3 weeks old and the water was very clear. It was filled using tap water and we have 12 small gold.The water was staying very clear until it rained for 3\4 days non stop and now has a tinge to it and is cloudy. Can rain water do this to a pond ???
    Your kind attention is appreciated.

  2. Avatar photo

    Hi Patsy, the tinge is probably overflow from the rainwater either spilling into the pond or if it was a heavy rain, then the rainwater would splash into the pond. The dirt will give it a slight color. Remember that ponds do go through cycles and they are not swimming pools. While, they should not be swamps by any means, they will have some growth. I suggest getting involved with a group on Facebook such as ​Watergardening and Koi/Goldfish Ponds


  3. Cathleen Kennedy

    I have three ponds. Each one empties into the other and within the last two weeks I found two of my fancy goldfish blown up and dead and the rest of them seem to be staying in one corner on the bottom.

  4. Avatar photo
    Lora Lee Gelles

    Here’s an answer for you Cathleen, from Richard Heimberger of Healthy Ponds:

    I do not encourage anyone to harvest rain water for fish keeping unless they understand water chemistry. In most cases, rain water is acidic (PH 6), has very little KH & GH, and contains high amounts of CO2 (carbonation). We can “cook” water with baking soda, calcium chloride and or magnesium chloride to make it fish safe. In your case, if these fish deaths occurred after a rain event, I’d test my water immediately. The isolating behavior and lack of forward mobility on the rest of these fish make me think PH is crashing or swinging. Again, test your water. Robust aeration in your ponds would be of help to your fish. Sodium chloride at .15% would be helpful for stress.

  5. Since a properly constructed pond is not subject to run-off, it would take a tremendous amount of rainfall to effect a major pH change.

  6. My Koi fishes looks so happy when it rains. They all go up to the surface and play like dolphins trying to get out from pound. Its like a mating spree !!!

    1. Please I am having a challenge with water management but we do have abundant of rain in my area, so what is the best material for collection of rain water to avoid this toxicity from roofing materials?

  7. Hello Victor,
    My name is Richard Heimberger. I am the owner/operator of Healthy Pond based in the suburbs of Chicago, Illinois. Realize that rain water is unsuitable for keeping fish. The PH is 6, KH is 15, and GH is 15. Rain water also contains 5-8 parts of CO2. Unless you are familiar with how to modify water for fish habitation, do not attempt this.

    Most plastic is fish safe and so is EPDM rubber pond liner. Since you will have a long term need for pond water, a rain water harvest system is in order. I would urge you to get your roof runoff tested to see if it is actually poisonous to fish. Most universities have an agriculture department with a soil and water test lab. If the roof runoff is good water, all you will need are some rain harvest barrels on the down spouts.

    1. I have a very large roof and have prepared a large underground cemented tank to harvest water. I thought I would have enough water for my two fish ponds in my compound to be operationalized soon. The roof is of baked (red) clay tile. Are these tiles okay for water harvest? Is there a way to treat this rainy water so that I can still use it? Which water source is best for fish ponds?

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