But just because tubercular bacteria are in the water doesn’t mean all or any of the fish have the disease. In humans TB is everywhere in the environment. It usually takes a person with a compromised immune system to manifest the infection. I assume that this applies to fish too.
In the confines of an aquarium, the Mycobacteria get shed into the tank by an infected fish and there's nowhere for the fish to go to avoid it. This means every fish in the aquarium will eventually contract it.
When the fish are infected, the Mycobacteria start growing in or on them (usually in them). When the bacteria grows on an organ like the liver or kidneys, the fish acts and lives a normal life until the bacteria has grown big enough and eventually damages the organ causing internal organ failure. At which time the fish bloats up overnight and dies shortly after that (usually within 24-48 hours). Depending on the overall health of the fish, the Mycobacteria can kill them faster or slower but there isn't much difference in time (maybe a month or two). The size of the fish and the temperature it lives in are the biggest factors relating to how long a fish can live with the disease for. Bigger fish live longer because they have bigger bodies with bigger organs and more bacteria are required to damage the bigger organs. Colder water slows the growth of the bacteria and that extends the life of the fish. But once a fish is infected, it's only a matter of time before they die from it.
Most fish coming out of Asian fish farms have been exposed to Fish TB either at the fish farms, at the exporter's facility, or at the importer's facility. If the fish don't pick it up there, they will probably catch it at the local pet shop.
When my fish had TB back in 2005 I did research into how common Mycobacteria was in fish and pet shops. Back then I contacted a lot of shops and importers in Australia and they all admitted they had TB in their tanks or assumed they had it in their tanks, either at the time or previously, some time in the past. They all knew about it but there wasn't much they could do except bleach their tanks if something unusual showed up. This helped a bit but unless you get disease free stock, you will never get rid of the disease. And disease free stock from fish farms (especially the Asian fish farms) is never going to happen.
With people, Mycobacteria is uncommon in some places but common in other places. Where it is uncommon (cities in well developed countries), it doesn't spread readily because fewer people get exposed to it, and if someone is identified with TB, they are isolated and treated and nobody goes near them without protective clothing. In areas where Mycobacteria are common (third world countries and overcrowded cities in some countries) the disease can spread readily because of poor hygiene, lack of medical care, and lots of people in close contact. This allows the disease to move from person to person and not get treated as it moves around society.
There are also multiple species of Mycobacterium. Some species affect people (TB in the lungs and leprosy on the skin) while other species affect birds, reptiles, mammals and fish. There's around a dozen species known to infect fresh, salt and brackish water fishes. Some of these can infect people and cause localised slow growing sores called granulomas that need antibiotics to treat.
