Sourcing

Free or very low cost on Facebook Marketplace

Marketplace is the single best place to kit out an aquaponics build for next to nothing. Set up alerts for these search terms and check daily, because the good free stuff goes fast.

Besser blocks

For your stands and bed supports. Search "besser blocks", "breeze blocks" or "concrete blocks". People give away leftovers after a landscaping or retaining-wall job all the time. Solid, free, and they take the weight of a full media bed without complaint.

Lumber

For stands, frames and bracing. Watch for "free timber" and "offcuts". If it is going anywhere near the waterpath or a grow bed, make sure it is untreated, because CCA treated pine leaches arsenic and copper that will harm your fish.

Old bathtubs

One tub becomes a media bed or a gravel bed, and a second-rate one still makes a great worm farm to feed the system. Search "old bath", "bathtub" and "cast iron bath". Renovators want them gone.

Old plumbing

Pipes, fittings and offcuts. Search "PVC offcuts" and "plumbing". Saves you a surprising amount at the hardware store. Just keep it clean and bin anything that has carried chemicals.

Gravel

For media beds. See the note below on blue metal, and avoid anything that fizzes in vinegar.

Almost free

Tote tanks (IBCs)

The backbone of this whole approach. One 1000L tote gives you a fish tank, a sump or a media bed depending on how you cut it. Buy food-grade only and ask what was in it before. Avoid anything that held chemicals at any price.

Blue barrels

200L food-grade HDPE drums are perfect for settlement tanks, mineralisation tanks and standalone grow setups. Same rule as totes: food-grade, and know the history.

Plumbing bits (when you need new)

Marketplace covers a lot, but sometimes you need a specific elbow, tee, tap or barbed fitting and you want it now. My goto for that is Dural Irrigation. They ship Australia wide and stock the irrigation and PVC range you actually want for a backyard build: angled bits, taps, valves, pipe and fittings.

Buying the bits is one thing; plumbing the system properly is another. Murray Hallam's course has some of the best content I have found on layout, siphons and the practical side of it all. Do the training before you start cutting and gluing a lot of PVC.

Gravel: use blue metal

I use blue metal, which I just buy by the square metre from a local landscaping supplier. It is cheap, it is everywhere, and because it is crushed basalt it is inert, so it won't dissolve and push your pH up the way limestone does. That matters more than people realise, because the wrong gravel will fight you the whole way through cycling.

You do sometimes see blue metal pop up on Facebook Marketplace too, usually leftovers from someone's landscaping or driveway job, so it is worth an alert if you would rather pay nothing.

Whatever gravel you use, run the cheap test first: a splash of vinegar on it. If it fizzes, it has carbonate in it and will mess with your pH, so leave it. Blue metal won't fizz.

Catch your own top-up water

I run 2 IBC totes off the shed roof gutter to collect rainwater. It tops up the aquaponics system as it evaporates, and it waters the soil garden as well, so I'm not paying for town water or putting chlorinated water near the fish. If you do use town water for a top-up, let it sit and aerate for a day or two first to gas off the chlorine before it goes anywhere near your fish.