Brisbane

Aside from ensuring the water is clean and there's good filtration, I'm not sure there's much you can do.

Interesting that the rainbows are all affected to the point of lesions and slowly dying one by one over the period of a few months so far, but the corydoras, plecos, and tetra populations are completely unaffected.

Bit of light reading:

https://aquariumscience.org/index.php/10-4-1-fish-tb-in-depth/

https://www.tfhmagazine.com/articles/freshwater/mycobacteriosis-the-stealth-disease213231