Plants roots can't pierce even PVC liners so I wouldn't let that limit your choice. It's a myth. Many people believe it's true because there have been a few, very few, pictures of roots growing thru pond liners. The actual cause is there already was a hole in the liner, which water leaked thru into the surrounding soil. I've seen pictures of pond plants growing thru the hole and also terrestrial plant roots growing into the pond. It's kind of like when a tap root in plastic pot grows thru a hole in the bottom and gets big enough to stretch the hole. It sure looks like the root punched a hole in the pot. But because it's a pot you know what happen. Roots grow at the tip like a wedge to get thru soil. The wedge has to make an opening in the soil for the root tip to grow into. The tip itself can't even push thru soil. The tip is very soft and delicate even on a bamboo. Also in the past few years some researchers found some interesting things about how roots grow. How they find water and nutrients and how roots know to spread out. In was documented in the show "What Plants Talk About". It explains why roots turn when the hit a liner or side of a pot. As an experiment to test the myth I once planted Giant Reed, Arundo donax,in a small (like 2'x2'x2') "pond" made of EDPM. Now Giant Reed I think makes bamboo (which I have grown) look like Pussy Willow when it comes to fast growth and the nastiest looking roots I've seen. The roots did not grow thru the liner. After a year or so it did become root bound and did push out the liner, like a root bound plant in a plastic pot. But the EDPM had no trouble stretching and handling being pushed very hard against the surrounding soil. That was as an extreme test as could imagine to disprove the myth.

I think you will probably not be very happy with a submerged filter. They are incredibly hard to clean and do not get the waste out of the water....it is just trapped in the filter box. You would be much better off with an external filter. I am attaching a link to a koi forum where there are lots of great examples of ponds and DIY filters. Maybe you can get some good ideas there. If you choose to buy a filter, remember that the manufactures stated pond size is always WAY off. If they say a filter is good for a 1000 gallon pond, it will probably struggle to keep a 500 gallon pond filtered. And I don't know if you have thought about this or not, but rocks in the bottom are VERY unhealthy for fish and require a lot of maintenance. All the uneaten food and fish waste settles into the cracks and rots. The rotting debris is a haven for parasites and harmful bacteria. A clean pond is a healthy pond and it is impossible to keep a pond clean with that gunk in the bottom. Now regarding fish. You probably would be happier sticking to goldfish. You might be able to get away with a couple of koi, but keep in mind that koi really need 300 gallons each with a minimum pond size of 1000 gallons. They get very large and produce a lot of waste. Once they get big, they are also very damaging to plants. If you do get koi, stick to the butterfly type....they grow a little slower and don't get a big as regular koi. Here is a link that might be useful: Koi shack

I've not seen Iris growing through liner, and I've seen some that should have. Bamboo WILL pierce liner. At least the running Bamboos. That's about it. I've serviced a lot of ponds, but I really haven't seen any plant punctures other than the babmoo. You could add a layer of Landscape Fabric to add another hurdle for any plant to get through, if you wanted. Greg

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