Feeding your plants with nutrients from fish waste is a reliable path to a strong harvest, whether you’re growing tomatoes or any other crop. Before your aquaponics setup can run smoothly, though, there are a handful of things you need to get right.
Understanding Aquaponics
Start by getting familiar with the individual components that make up an aquaponics system.
Fish Tank
You’ll need a dedicated space to house your fish. If you’re running floating rafts, keep the fish tank separate from the growing area — fish that can reach plant roots will eat them, stunting growth, and solid waste settling on roots can block nutrient uptake and cause rot. Splitting your fish across multiple tanks also lets you stagger your harvests; the UVI system, for instance, runs four separate tanks and harvests fish every six months, which keeps nutrient supply to the plants steady. Harvesting your entire fish stock all at once, by contrast, causes feeding rates to drop sharply, which cuts the nutrients reaching your plants.
Pump
Water typically flows from the fish tank to the growing area by gravity, then continues to a sump tank before being pumped back to the fish tank. Choose your pump size carefully based on your system’s needs, and plan to circulate the entire volume of your fish tank at least once every hour.
Filter
A dedicated filter isn’t strictly required if your growbeds are sized correctly, since the growbed media itself will break down solid fish waste into a form your plants can use. If your media isn’t sufficient to handle the solids, you have three options: add a filtration system, expand your grow media area, or cut back on feeding (which also means lowering how many fish you stock).
Grow Media
In a media-bed setup, plant roots grow directly in whatever grow media you choose. If you’re running deep water culture (DWC) or nutrient film technique (NFT) instead, you’ll need to add separate biofiltration to handle solids processing.
Working Out Your Feeding Ratio
There’s no universal fish-to-plant ratio that works for every setup — it depends heavily on which growing method and crop you’re using. As a starting point, match your feeding rate to your growing technique:
- Deep water culture (DWC): 60–100 grams of feed per day for every square meter of growing area
- Growbeds: 15–40 grams of feed per day for every square meter of growing area
- NFT: 15–25 grams of feed per day for every square meter of growing area
You also want to keep solids discharge to a minimum by running a solids filtration system — letting solids pass through undigested means less of that waste gets broken down into nutrients your plants can actually use. Mineralization is the process that turns fish waste into a plant-available nutrient form.
Working Out Your Ratio for Growbeds
Here’s a worked example. Say you’re running three growbeds, each 4 feet by 4 feet with a 12-inch depth, filled with hydroton as the grow media. At 15–40 grams of feed per square meter per day, and a combined grow area of 3 square meters across the three beds, your total daily feed works out to somewhere between 45 and 120 grams. Use the lower figure, 45 grams, if you’re not discharging solid waste, and the higher figure, 120 grams, if you are routing solid waste out to your garden. Feeding at this rate should keep leafy greens free of nutrient deficiencies; heavier feeders like tomatoes will need more, closer to the 40-gram-per-square-meter end of the range.
From there, you can work out how many fish it takes to eat that 45 grams of feed per day. Tilapia grow quickly and convert feed efficiently, with a feed conversion ratio (FCR) of about 2 — meaning it takes two pounds of feed to produce one pound of fish — and they typically reach a full pound in about six months from fingerling size.
Multiply 45 grams by 182 days (roughly six months) and you get 8,190 grams of feed needed over that period, or about 8.19 kilograms. Converting that to pounds (dividing by 454) gives you 18 pounds of feed. Divide that by the FCR of 2, and you get 9 pounds of fish supported by that feed. If you’re raising fish to a one-pound harvest weight, that’s 9 fish (9 ÷ 1); if you’re taking them to a pound and a half each, that’s 6 fish (9 ÷ 1.5).
Water volume matters too — tilapia need about 2 gallons of water per pound of fish, though this varies by species (trout, for example, need closer to 8 gallons per pound). For the tilapia example above, you’d need a fish tank of at least 18 gallons. Tilapia actually tend to do better in tanks stocked more densely, since they chase each other less and experience less stress as a result.
Working Out Your Ratio for DWC or NFT
If you’re running deep water culture or NFT instead of growbeds, follow the same approach but swap in the feeding rate for your system — use the DWC or NFT figures from the feeding-ratio ranges above in place of the growbed numbers.
The bottom line is that there’s no single fixed fish-to-plant ratio that applies everywhere. What you should actually use depends on:
- Which growing system you’re running
- How much grow area you have available
- What kind of plants you’re growing — leafy greens versus fruiting crops
- Your water temperature, since it affects how well nitrification is working