I'm thinking about making a waterfall

[tubeN2]

Those plastic pieces look like holders for the lava rocks or other media that you may put in them to help grow the beneficial bacteria and enzymes.

Some of the ponds that I have built have filter media sheets and permeable bags with the lava rock inside to hold the media from free floating off. Others have the media in smaller containers such as your plastic pieces in photo 3. The heavier the media and rocks means the more flow of water you can have without loosing your biofalls.

Lighter media requires less flow so that the filter media will remain stable and not float away. [fishon]
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