Feeding fish and setting stocking levels in a small system

In aquaponics, fish food is the fertilizer. Nearly all the nitrogen the plants receive starts as protein in the feed, passes through the fish as ammonia, and reaches the roots as nitrate. That makes feeding the main control you have over the whole system. Too little and the plants go hungry, while too much pushes ammonia up faster than the bacteria can clear it, and overfeeding is the usual way a small system that was running well gets into trouble.

How larger systems balance feed and plants

Backyard and commercial growers size their systems with a feed rate ratio, measured in grams of fish feed per square meter of grow area per day. The FAO manual on small-scale aquaponic food production suggests 40 to 50 grams a day for leafy greens and 50 to 80 grams for fruiting plants. The idea behind the ratio is that the feed and the plants are matched to each other, and the fish are how the feed becomes fertilizer.

At desktop scale the numbers get too small to weigh usefully. A grow bed measuring 30 by 20 cm covers 0.06 square meters, which works out to two or three grams of feed a day for leafy greens, far more than a betta or a handful of nano fish will eat. Small systems are underfed by commercial standards, and that's fine. It's the reason they suit herbs and leafy greens better than tomatoes and peppers, and why nitrate in a desktop system tends to stay low.

How much to feed small fish

For small aquarium fish, feed what they'll finish in two to three minutes, once or twice a day. Judge the amount by what's left over. Anything still drifting or sitting on the bottom after a few minutes is excess, and it should come out with a small net or a turkey baster before it breaks down. Many keepers skip one day a week, which does the fish no harm and gives the bacteria a lighter day.

Use a good-quality food sized to the fish's mouth, such as a small pellet for a betta or a crushed flake or micro-pellet for nano fish. Most aquarium fish will eat well past what they need whenever food is offered, so their appetite is a poor guide to how much the system can handle.

Adding fish in stages

Bacteria grow to match the ammonia they receive. A newly cycled system has enough bacteria for the ammonia it was cycled on, and a full load of fish on the first day can still overrun it. Add about half the planned fish, feed lightly, and test ammonia and nitrite every couple of days for two weeks. If both stay at or near zero, add the rest.

The same staged approach applies to any later addition, and to raising the amount you feed. If you've just finished cycling, our guide to cycling a new system covers the water change to do before the first fish go in, and our guide to choosing fish has sensible stocking levels for 3 to 10 gallon tanks.

Signs a system is overloaded

Ammonia or nitrite above zero in an established system is the earliest and clearest sign. Other signs tend to follow. The water turns cloudy or smells sour, uneaten food builds up in the corners, nitrate climbs week after week despite healthy plant growth, or fish spend time gasping at the surface. That last one can mean low oxygen as well as ammonia, so check the pump and aeration while you test.

Cut feeding first, since it takes effect within a day. If ammonia or nitrite reads above about 0.5 ppm, do a 20 to 30% water change with dechlorinated water and retest the next day. If the readings creep back up once feeding returns to normal, the system has more fish than its grow bed can support, and either some fish need a new home or the grow bed needs to get bigger.

Feeding while you're away

Healthy adult fish in an established system can go a few days without food, and the water is often more stable for it. For a long weekend, skip feeding altogether. For longer trips, a friend with pre-measured daily portions is safer than an automatic feeder, which can jam or dump several days of food at once. Slow-release vacation feeder blocks can foul the water in a small tank, so leave them on the shelf.

When the plants want more

If nitrate stays near zero and the plants show the pale older leaves of nitrogen shortage, the fish load is too small for the grow bed. Add a fish or two, or raise the feeding slowly, using the staged approach above and watching the test results. Our guide to plant nutrient deficiencies covers the other shortages that can look similar, and the article on top plants for filtration efficiency explains how to read basil and lettuce as a running gauge of the balance.