When I built my backyard system, I planned to use marine grade plywood with pondliner (or have it coated with fiberglass). And then I did the math and the cost was hard to justify. That’s why I opted for Intermediate Bulk Containers (IBC) tanks.
Here’s a tip, many cut the bottom and the top portion for a more or less squarish grow bed 12 to 14 inches deep. But if you cut it from the sides, you’ll have a more rectangular-ish end product with a bigger surface area that means you can grow more crops if you opt for this approach.
Best of all, no need for pondliners, no need for fiberglass, and the grow bed you end up with is sturdy (there’s even a metal support cage), lightweight and already waterproof!
And it costs less, at least back then when aquaponics wasn’t an “in” thing yet and IBC tanks were cheaper.
I used these IBC cutouts for my flood and drain media beds.
About media beds, the usual package you can buy in the market is the IBC chop and flip system. Since the number of those who sell complete setups have grown during the pandemic, the competition is fierce, especially with pricing.
As a consequence, I noticed that the bottom of the media seems to be thinning. Maybe it’s the cost cutting measure. The standard is 12 inches deep of media, this let’s you have two inches top dry zone, six to eight inches wet and dry zone (due to the flood and drain mechanism), and then last two to four inches at the bottom where the fish waste accumulates and undergoes re-mineralization.
I see IBC chop and flip systems with just around 6 inches of media depth. Less media means less solids filtration, less biofiltration, and this shallow depth also affects the dry, flood and drain, and bottom zone of the grow bed.
It also results into an additional problem. Roots don’t like solid fish waste lumping on it and thus reducing it’s ability the absorb nutrients. What you get is root rot and the plant dies. Hence, systems that use the roots of plants as the “solids filter” eventually crash and fail. This is a common mistake of those who are in NFT aquaponics, they designed it in such a way that the plant roots will be the solids filter.
If your media bed is 12 inches deep, this is less of a problem because you have those three zones and the always wet portion (where the solids accumulate) is at the bottom two to four inches off the system. This means that your crops will have several inches (around six to eight, or even 10 if you count the top two inches) to grow generally unperturbed by solid wastes that are at the lowest level.
If your media bed is just six inches deep, well, those solids build up fast and will become a problem, and this will eventually result into more problems for your crops and your aquaponics system. Err on the side of caution. As much as you can, follow the 12 inches deep media bed rule of thumb.