I have fiber glass walls in my viv I'm building and for my waterfall it will be styro foam coated in grout then painted and sealed with mod podge. It can even hold water and is cheap and light but strong.
My fiber glass walls were premade sheets from home depot so different then what your talking about. And mine will be like 80% covered. You can look at my threads if you wanna see sturo foam rock tho for your waterfall. Wish I could show you the actual one but I haven't built it yet.
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