Before modern rubber fish pond liners became available, people would just use concrete. Without a doubt, concrete is strong; but it tends to crack over time. Even the slightest earthquake, or ice pressure in winter can crack concrete. The new fish pond liners are made of the kind of synthetic rubber called Ethylene Propylene Diene Monomer, or EPDM. You get sheets of these to line your pond with, and they stretch over any unevenness that there may be. If there is ice pressure or a slight earthquake, these rubber liners just stretch to accommodate it all. when it comes to durability, you might be surprised to know that EPDM can be as tough as concrete.
Perhaps people are surprised to hear about how durable it can be, because they think when they hear the word “ethylene”, that it must be something like polyethylene – the ubiquitous plastic that we find in it everything, everywhere. In truth, the two have nothing to do with each other. EPDM is a completely different thing and it’s very tough.
You could actually use fish pond liners to build a pond above ground just as you can build something more traditional below ground level. But there are specific ways you do need to follow instructions for how the pond is to be shaped. Let’s say that your plan is to build a belowground pond. You need to make sure that it is at least 2 feet deep, that the sides are straight, and that the floor itself is slightly sloped to enable water drainage.
EPDM can get really tough and inflexible when it’s cold. It’s usually a good idea to leave it out in the sun so it can soften. That way, when you lay the liner down, it will hug all the curves of your pond and really fit snugly. Snugness is pretty important; if you can’t get it as snug as you would like it, you can be pretty sure that once you fill your pond with water, that the weight of the water will settle everything nice and tight.
Now with fish ponds, the liner does need to the fish safe. When the synthetic rubber comes into contact with the water, it’s possible that there will be a certain amount of water contamination that will take place. Most experts recommend that you wait for some time after you fill your pond for the first time. You need to let the water heat up in the sun, so that any chemicals that the liner may leach out, do manage to come out. After that, you can change the water out completely, and then, your pond should be ready for fish.