Fish waste isn’t the perfect fertilizer. Waste is mostly ammonia, which is nearly useless to plants and deadly to fish. The perfect fertilizer is nitrate. The ammonia in fish waste becomes nitrate when enough beneficial bacteria have done their magic. "Media,” like rocks and hydroton balls, are where the beneficial bacteria like to live. It so happens that “media” does a nice job of physically supporting plants and roots.
When it comes to media, I stand on the shoulders of giants. Those who have gone before me assert that you need something that is ¾” in diameter, on average. Dirt, sand, and small pebbles will clog and develop oxygen-deprived zones when used as the predominant medium in an aquaponics system.
What is the best media? It depends. The major options are gravel, river stones, hydroton, and expanded shale /slate.
- Gravel is cheap, maybe $10 for 50 gallons. But it’s heavy and hard on the hands. Depending on where it comes from, it may not be pH neutral.
- River stones aren’t quite as hard on ones hands as gravel, and are also cheap. They are also heavy and may not be pH neutral.
- Hydroton is the lightest option, it’s pH neutral, and it’s a dream to plant in. But Hydroton is only made in Germany and China, and it costs about $200 for 50 gallons (sold in 50L bags), assuming you can find it locally and don’t have to pay shipping. One downside to Hydroton is that it will shift under pressure, like when a heavy wind blows though your garden.
- Expanded Shale and Expanded Slate are pH neutral and almost as light as Hydroton. These products are both made in the US and are primarily used in construction and landscaping. If you can find a local bulk supplier for the ¾” product, the price per unit volume can be as inexpensive as gravel. But if you have to get it shipped, it is about as expensive as Hydroton.
I’ve used river rock and Hydroton, and I loved the benefits of Hydroton. When I moved to my outdoor system, I divided the river rock from my single initial growbed across the four growbeds, filling the top with Hydroton. I figured the expensive loveliness of Hydroton would be wasted in the bottom-most sludge region of my growbed. And perhaps that layer of river rock anchored plants that might otherwise have toppled.
If you’ve got enough media hosting beneficial bacteria, you can consider converting one of your growbeds to constant flood (with or without floating raft). The rule of thumb I’ve seen for home systems is 3-4 media beds for each constant flood bed. Constant flood beds can be good for starting seeds, growing duckweed, or growing leafy plants like basil and lettuce.