You're right, columnaris is a bacterial infection. So it would require even less exposition tu uv to kill.
But if I get a uv steriliser, I might as well get one that can also kill most parasites.
I didn't test my water in years, but I'm pretty sure the problem is not from bad water quality.
My 25g has been running for years. I've had my 2 corydoras for 10 years. 10 lampeye killies (7 now) and 3 dwarf frogs for 5 years, 6 amano shrimps for 3 years.
No death in the last 3 years, except this summer a betta i've had for almost 4.5 years, and a frog when I moved 3 months ago.
So 2 weeks ago was the first time I get new fishes in 3 years, a couple of dwarf gouramis. The very first day they scratched against plants, and the male acted almost like a zombie. To make the story short, the male died 1 week after. And then I saw the female had something that looked like a scratch close to the mouth, which now looks more like columnaris. And 3 killies died since. The female gourami is now very weak and I wouldn't be surprised to find her dead when I come home later. Yesterday I put her in a bucket hoping to stop the infection from killing other fishes.
So yeah, I'm pretty sure the gouramis intruduced an infection in my aquarium, and I don't want it to happen ever again.
For now I'll get medications, but for the furure I'm hoping a uv sterilizer will prevent this kind of thing.