Backyard Aquaponics
A place to discuss aquaponics

I hardly need to follow your 'workings' in order to make some relatively simple mathematic calculations, Trout. You have missed my point: bottle top sizes are either smaller now than when you made your measurements, they are a different size in whatever state you live in compared to those produced in WA (where Sleepe is located), or you have measured incorrectly.

Sleepe, I suggest that you measure your particular brand of bottle top since they may be different again from Trout's and mine. Also, I would at least double the figure that you calculate for the area of the cylindrical sides to allow for the extra surface area of the internal and external convolutions. I have been collecting soft drink bottle tops since I read your thread and now have hundreds of them (none of which match trout's specs, BTW),so thanks for the idea. I also have a growing collection of milk bottle tops for the same reason - I see them as a potentially useful adjunct to a range of filter types. I think they will get a run during the coming trout season.

I don't belive they'll equal, or even get close to the surface area of media, bio-balls or K1....

But hey .. they're free... why not...

Another option...but not for moving bed. Is to contact your local plastics manufacturer, or one that works heavily in customs fit outs for food production/service/equipment and ask them if you can have a bag of their HDPE lathe swarth.

Alternative, if you have the time, and an electric plane, take to a blue hdpe barrel with the plane and make your own swarth. If you had a sharp enough blade, you could probably fashion something from and old, geared down electric motor to spin the blade and shave the rim diameter of the HDPE. Imaging an upside down food processor/slicer for a blue barrel.