EPDM Revisited

A reader takes exception to the negative assessment of this material for fish ponds.

Q. I am writing this letter in reference to your claim regarding the suitability of EPDM as pond liner material. While you claimed that it was toxic to fish, my experience has been just the opposite.

I have two ponds in which I use EPDM rubber. One holds 1500 gallons (5700 liters) and has been in operation since 1988. The other holds 5000 gallons (19,000 liters) and was installed in April 1990. My koi in both ponds have spawned many times. The fish happily graze on the algae growing on the liners. I think you gave EPDM a “bad rap.”

A. Eugene, I appreciate the time you took to raise this issue again. Your letter makes a point that highlights one of the biggest problems in the area of pondkeeping. Although practically all aquarists use products designed explicitly for rearing fish, many pondkeepers prefer to improvise and use products marketed for other purposes, such as general construction. And when they do so, they unknowingly take risks.

Liner materials are a good example. Despite the existence of a variety of certified fish-grade liner materials, pond enthusiasts continue to search for cheaper, stronger or thicker materials. I think this desire to innovate is terrific, but there is a price to pay — often the lives of the fish.

Many beginning pondkeepers are shocked at the price of fish-certified liners, especially compared with the price of standard swimming pool liners. As a result, they frequently try to build a pond on the cheap with nice aqua-blue swimming pool liners. In most cases, these folks find their fish floating belly-up within 24 hours because the vast majority of swimming pool liners are impregnated with algicides, fungicides and other chemicals that are toxic to fish.

I say vast majority because there are a very few swimming pool liners that are perfectly safe for fish, but it is virtually impossible to tell ahead of time which ones are safe! Thus, when you try to use a swimming pool liner for a pond liner, you are playing Russian roulette with a fully loaded gun.

What about EPDM? There are several manufacturers of EPDM material, and each has introduced a slight variation. One states categorically that their product is indeed safe for fish, whereas another states equally emphatically that their product is guaranteed to kill fish and they warn their sales staff to be sure to inform potential customers of this fact. I do not know the situation with yet a third producer.

The unfortunate fact is that just as with swimming pool liners, there is no way for a pondkeeper who goes to the local building supply store for liner material to distinguish between the EPDM liner products that are safe and those that are not. Thus, for me to suggest that hobbyists use EPDM is to guarantee that one-third to two-thirds of those who do so will kill their fish. As I said, for every case like yours where safe EPDM was used, I know of more than a dozen in which the liner used was clearly toxic.

There is, however, some good news for EPDM enthusiasts. Lilipons — which has a nice mail order catalogue — has apparently begun to market fish-grade EPDM material. They claim to have extensively tested the material they sell, and because they back their liner as being non-toxic to fish, hobbyists can be assured that they are not purchasing one of the toxic versions. Some smaller mail order pond supply companies also sell EPDM liners now, and one has to assume they have tested these liners for safety. However, I would insist on a written guarantee.

The bottom line is that the majority of EPDM material sold as building material is toxic to fish. Consequently, I cannot recommend buying EPDM material at lumberyards, building supply houses and so on because I do not know of any way to tell whether the product being sold is safe or toxic for fish. Those pondkeepers who want to risk using EPDM material purchased at these types of retail outlets should try to elicit a guarantee in writing from the seller or the manufacturer. Buyer beware!

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