Usually tropical fish farming are done in open area fish ponds to produce high quality grade and large number of different types of fish. Ponds are preferred because it has large space for the fish to grow and although it requires a lot of maintenance, generally water parameters are much easier to control. The water is actually extracted from natural sources like underground supply coming from wells and also uphill streams so that superior water quality can be obtained. Basically, there are also a daphnia ponds to supply and provide natural live foods so that the fish can grow very fast to be colorful and healthy. Here the daphnia are actually grown and cultivated for the fish.
One of the most important aspects to ensure successful tropical fish farming is that there must always be a green house, which acts like a temperature control center. The water is pumped to a centralized location whereby there are aquarium chiller units, heater and filters, which function to regulate a certain water temperature and then the water is filtered before releasing back again to the pond. Sometimes, these fish breeders also use large tanks kept in warehouse for storing certain high quality fish like discus and arowana while goldfish, guppy, neon tetra, catfish are all kept in outdoor ponds. These types of warehouses are constructed with skylights to allow natural sunlight to penetrate through to the aquarium tanks. Sunlight is important for healthy growth of fish to bring out bright colors.
If you get the chance to actually visit a farm, I would suggest you spend the time and witness the whole setup and how these fish suppliers work to mass-produce and overall care for the tropical fish. It’s exciting to learn and see with your own eyes how the fish are bred before they actually arrive in aquarium stores.