I think you are going to be ok with common plecs in regards to your liner. I have some full grown ones and I have seen them in outside ponds with liners. I realize how "spiney" they are, but I just don't see them managing to pierce a pond liner even of average thickness ( which I would advise whatever liner you are looking at bump the thickness up one notch, it'll be worth it.) I'd be more worried about walking around on it and pointy,sharp decorations like rocks and such falling over. just my .02