Don't buy medications, a UV steriliser or new filter because there is no cure for fish TB. Just save your money and wait till they die, then scrap/ sterilise the tank with bleach and start again.
The Mycobacteria have a waxy coating over them and this protects them from medications, drying out and dying quickly. They have found live but dormant Mycobacteria in the bottom of dry river beds that have not had water for years.
Mycobacteria are an ancient species of bacteria and cause Tuberculosis in humans, animals, birds, fish and reptiles. They also cause leprosy and are found on the fangs of white tail spiders. If bitten by one of these spiders, the tissue starts to rot away and you need anti-biotics (or in my case honey) to treat it.
The best you can do is feed the rainbows lots of plant matter and remove any frozen bloodworms from their diet. Then let them live out their lives until they show major symptoms (stop eating, swell up overnight, do a stringy white poop, hang out at the surface or by the filter outlet gasping). If you see any with those symptoms, euthanise them immediately because they are dying from major organ failure.
Reducing the temperature can help prolong their lives as it slows down the bacteria that cause this and they don't grow as fast, but they still continue to grow.
