Tilapia is the fish species most commonly cultured in aquaponic system. Although some aquaponic systems have used channel catfish, largemouth bass, crappies, rainbow trout, paccu, common carp, coi carp, goldfish (Carassius auratus),asian sea bass (barramundi) and murray cod, most commercial systems are used to raise tilapia. Most fresh water species, which can tolerate crowding, will do well in aquaponic systems (including ornamental fish).
Goldfish are popular pond fish, since they are small , inexpensive, colorfull and very hardy. In an outdoorpond or water garden, they may even survive for brief periode of ice forms on the surface, as long as there is enough oxygen remaining in the water and the pond doesn’t freeze solid. Common goldfish, London and Bristol shubunkins, jikin, wakin, comet and some hardier fantail goldfish can be kept in a pond all year round in temperate and subtropical clomates. Moor, veiltail, oranda and lionhead can be kept safely in outdoor ponds year round only in more tropical climates and only in summer elsewhere.
Koi are domestically common carp (Cyprinus carpio) that are selected or culled for color, they are not a different species, and will revert to the original coloration within a few generations if allowed to breed freely. In general, goldfish tend to be smaller than koi, and have a greater variety body shapes and fin and tail configurations. Koi varieties tend to have common body shape, but they have greater variety of coloration and color patterns. They also have prominent barbels on the lip.
Some goldfish varieties, such as the common goldfish, comet goldfish, and shubunkin have body shapes and coloration that are similar to koi and can be difficult to tell apart from koi when immature. Since goldfish and koi were developed from different species of carp, eventhough they can interbreed, their offspring is sterile. To recover high capital cost and operating expenses of aquaponic systems and earn a profit, both the fish rearing and the hydroponic vegetable components must be operated continuosly near maximum production capacity. The maximum biomass of fish a system can support without restricting fish growth is called the critical standing crop. Operating a system near its critical standing crop uses space efficiently, maximize production and reduces variation in the daily feed input to the system, an important factor in sizing the hydroponic component.