Everything's going wrong!

I have a 10g (38L) planted freshwater tank containing 1 betta, 5 cherry barbs and 4 pygmy corys. It's kept at ~26-27C (79-81F) by an adjustable heater, and until last night it was filtered by an Elite Stingray 10. Something came off the filter last night, making it loud enough to wake me up. I turned it off, and after a trip to the LFS I had replaced it with a Fluval Mini 6 hours later, which I am delighted with. My tank no longer resembles a whirlpool. I used all the old media.

One of the corys, I think, has columnaris. I got some Melafix for it. I gave the second dose today treating the whole tank. I'm fairly sure it's spread to the betta, because when I got up to turn off the filter I noticed that a large amount of his fin had turned black and had rotten off, which I hadn't noticed 8 hours earlier. The other fish appear to be fine. Does Melafix do anything? I know the recommended treatment is salt, but it'd kill the plants and the corys. Also I'm in the UK, so I'd have to import Maracyn 2, but if that's what it takes I will.

I also have an ongoing algae problem, but I think that was caused by the knackered light, which I have replaced. And once I have the columnaris cleared up, I'll get some shrimp.

In conclusion: Columnaris - Help! Many thanks in advance.

EDIT: The sick cory was sat in the perfect place on a leaf so I got some pics.