Advice for introducing new fish or mixing Discus of different strains ( origins / farms).

The mixing of strains is absolutely not doomed to failure, everything depends on the strains.
This mixing is done all the time without even knowing it, when you buy a plant or other small fish there is also a risk of mixing bacterial strains but often without consequence.
The problem is that many Discus farms practice systematic antibiotic treatments as a preventive measure throughout the breeding process and produce immune Discus and healthy carriers of ultra-resistant bacteria because they are used to antibiotics.
This phenomenon is particularly harmful with Discus that stay for months at the farm.
While the risk is less important with plants or other fish because the treatments are less and the bacteria are more common and your Discus can fight them.
First of all you should know that if you mix your Discus and some of them get sick while all of them were in good shape, it is not the fault of those who get sick but it is the fault of those who are not sick because they are healthy carriers of disease or bacteria that the others are not immune to.
So, unless a Discus needs to be isolated to eat better for example, there is no point in quarantining your sick Discus and treating them separately without treating the others.

You have to treat all your Discus, even the ones that are not sick, because in the end all of them will get sick, even the healthy carriers that do not show any signs yet.
The first ones who get sick (not immune) develop the disease quickly and the healthy carriers who are not immune to such a large amount of disease, will get sick later.
So that’s why you have to treat them all.

To check the compatibility of your fish, put your new Discus in a separate quarantine aquarium from the old ones without any contact or sharing of equipment such as a landing net for example.

After 1 month and you are assured of the health of your Discus, two possibilities:

1 – Put the Discus you prefer less of the old ones and place it with the new ones, you then risk in the worst case to lose that Discus or all the new ones.
Or place a new one with the old ones, then you risk in the worst case to lose this new one or all the old ones.
If after 2-3 weeks all is well, they are compatible, otherwise treat all your Discus, new and old, even the ones that are not sick.

2 – Make water changes of 10% of your quarantine aquarium with water from the main aquarium and the reverse, from the quarantine aquarium to the main one. Increase to 20% after one week and 50% on the third week. If one of the two aquariums gets sick, you should not only treat the sick ones, but all your Discus, even the non sick ones, which are then healthy carriers.
This last method has the advantage that the Discus can become immune and develop immune protections against the bacteria of the others by putting them in contact progressively.
But it can also be inefficient because as I said at the beginning, some bacteria are ultra-resistant.