It's all about the fish
Clare Gill
The engine driving our aquaponics system is a 2200-gallon tank of goldfish. The previous owner had chosen goldfish, and none of our reading convinced us to make a change. The take home message from the research studies we read was that aquaculture (raising the fish for sale or consumption) is not profitable in an aquaponics system. We just want a fish that makes for a durable engine and requires little maintenance. Goldfish seem to fit the bill!
Goldfish tolerate the very wide range in temperatures that we have in central Texas. Because the fish do not need heating over winter, our energy costs (to run the water and air pumps) are consistent throughout the year (and under $3 per day). In winter, on our coldest days (lows in the 20s) the water temperature was in the 40s. Now in August, as we enter our first stretch of consecutive 100F days, the water temperature is in the high 80s. The only obvious difference between the seasons is their feeding behavior. We feed the fish 3 lbs of soy-free, 32% protein content, catfish food a day. In winter, we split this between three feedings, which they consume in 10 to 15 minutes. In summer, they demolish this in two feedings.
We can control the amount of ammonia excreted by the fish by the amount we feed them. Ammonia is converted to nitrite and then nitrate by nitrifying bacteria that have formed a biofilm on the rafts and sides of our system. If we were feeding more than the bacteria in the system could convert to nitrate, then we would see ammonia levels accumulate in the fish tank as the water recirculates. And the kicker is that ammonia is toxic to the fish. We test our ammonia, nitrite and nitrate levels once a week at the same time of day using test strips. Usually, the system has < 1 ppm ammonia.
This spring was very, very wet in central Texas. We’ve never experienced one like it! With heavy rain, we experienced frequent power outages. Although frequent, they didn’t usually last for long. If they reached 30 min, we would start a backup generator to run the air pump. Even on the worst days, we didn’t have much death loss – only one or two fish (often fingerlings). On top of this, we had some issues with our water pump overheating and cutting out. On days when water wasn’t moving in the system, either because the power was off or the water pump was off, we reduced the amount we fed or didn’t feed at all. This reduced nutrients available to the plants, but kept ammonia at manageable levels for the fish.
All in all, we’re very happy with our choice of goldfish. But they’re not pets and don’t all have names like the rest of our menagerie! Just a big engine that hums along very nicely, thank you.