The exact requirements vary among fish species, but each one requires a specific amount of feed, water temperature, stocking density, growth period and dissolved oxygen levels. Offering the wrong conditions to a fish species or mixing together two or more species with drastically different culture requirements, will eventually lead to catastrophic losses. Even if the conditions don’t immediately kill off an entire system full of fish, raising them out of their optimal range will result in slow growth rates, poor feed conversion ratios and increased disease rates. Because each tank of fish can cost hundreds or even thousands of dollars to replace after a loss, it’s best to match the fish species to the conditions you can provide most easily rather than trying to manipulate conditions with heaters and intensive water conditioning routines.

Water Temperature

As long as winter temperatures either stay naturally above freezing or you can provide heating and greenhouse cover to prevent these dips, you can grow fish in an aquaponics system year round. Fast-growing fish are a good option if you prefer to only raise fish during the warm months and want to finish multiple harvests in just a few months of warm weather. Heaters may sound like an easy way to keep water temperatures steady year round, but they can have many issues such as:

• Malfunctions that lead to overheating or a lack of heating; killing off fish either way.
• High electricity costs that decrease the profits of every fish harvest.
• Routine equipment replacement costs; as commercial scale aquarium heaters only last a few years.
• Circulation issues leading to hot and cold spots that kill off fish that can’t reach water with the appropriate temperature.

Choosing fish that thrive in the natural temperature ranges of your proposed site reduces or eliminates the need for water heaters. Greenhouses can further protect the fish tanks and plants from temperature swings, making the most of any supplemental heating that can’t be avoided. Fluctuations of more than three degrees in a single day can kill off even the hardiest fish. Air temperature changes don’t immediately change the temperature of a few hundred gallons of water, but without insulation and backup heating, a sudden cold snap can lead to thousands of dollars’ worth of losses.

Stocking Density

The stocking density of a fish species refers to how much crowding the fish can handle before experiencing growth issues. Packing too many fish into a certain size tank results in damage and death of fish stock.  When density is too high, fingerlings can attack one another or smother the smaller fish. Stocking densities in aquaponics are based on more than just the size and growth rate of the fish species, which are the primary considerations for traditional aquaculture. Since aquaponics systems must maintain a specific level of dissolved nutrients to support plant growth, the stocking densities for these systems must also take these nutrient rates into account.

Growth Period

Adding two species with similar requirements to the same tanks can work well, but there’s often too much competition if they’re identical in needs. Mixing two species that are the same in everything but growth rate results in reduced competition and a double harvest window from the same tank without any extra work. Make sure the fish share similar feeding needs before combining them in the same tank. This is imperative so that the slower-growing fish are not starved out by more aggressive and faster-growing competitors.

Dissolved Oxygen

It’s a common misconception that fish breathe water. Their gills actually extract the oxygen dissolved in healthy water. So, losing that oxygen due to a sudden algae growth, unintended spike in nitrogen levels or long periods of high water temperatures, can kill fish. You might think that all fish would need the same general levels of dissolved oxygen, but even this requirement varies quite a bit among species. Aerators are the best tool for adding oxygen to tanks and troughs used in aquaponics, but adding live plants that generate oxygen may work for some large outdoor tanks. Deeper tanks with smaller surface areas can reduce the amount of oxygen lost through evaporation and gas exchange. Yet, this leads to a separation of oxygenated and depleted water known as stratification. Because each type of fish tends to prefer a certain height in the water, a loss of oxygenated water at the bottom of a deep tank can wipe out bottom-resting species. Balance tank depth and surface area to promote oxygen gain through surface rippling while minimizing oxygen loss from too much exposed water.

Feed Conversion Ratios

It’s tempting to try and choose a fish species by feed conversion ratios alone. It’s true that a fish that generates as much weight as possible per pound of food reduces costs significantly. However, don’t forget to factor in the extra costs of keeping a species that isn’t right for your existing conditions. A fish with a high feed conversion rate that requires extensive year-round heating and specialty feeds are not worth it. A less effective species may turn out to provide a much more reliable profit, with lower upfront costs, resulting in more business income in the long run. Don’t forget that some fish with high feed conversion rates and wide ranges of temperature tolerances are also banned by many states for being potentially invasive if they escape into native waterways. Tilapia is one example.

Water pH Level

Many fish species used for aquaponics can handle a surprisingly wide range of water pH levels between 6 and 8, compared to more delicate aquarium species that can often only handle within a few points of 7. However, there are still plenty of types of trout that will stop growing or start dying off due to relatively small changes in the pH level. Because pH can change quickly and takes much more time to adjust back to the desired range, it’s essential to check these rating regularly. It is imperative to adjust often, and early, rather than waiting until there’s a serious problem.