By purchasing your pond components from a reputable distributor, like Keystone Hatcheries, you will be able to avoid using products that are unsafe for fish and other aquatic life. An example would be Pond Liners. Our Pond Guard Liners, made by Firestone, are specifically designed for use in fish ponds. A very similar E.P.D.M. roofing liner is available for less, but it leaches toxic compounds into the water, which can harm fish and plants.
However, there are other things to consider that you don’t always buy from a pond store. When building your pond, never use metal components unless they are Stainless Steel or Titanium. You can get by with some Aluminum components but always avoid Copper, Brass & Galvanized. These materials give off Heavy Metal ions (mainly copper and Zinc) that are very toxic to fish. An ingredient in water conditioners called E.D.T.A will neutralize heavy metals that get in via water lines, but if there are metals containing copper or zinc exposed in your pond, the accumulation could exceed what the conditioner can absorb and a fish kill could result. Also, improperly treated concrete leaches alkaline compounds into the water that increases the pH and can cause Alkalosis. If you are not sure of a building material, ask a pond professional.