help with bacterial infection - fish dying, no signs!

[wazzie]

Hi guys, I need a bit of help.
I have a platy tank (260l) with about 70 adult fish. It's been running about 6 years, no major problems. Water parameters are good, no change in temperature/diet, nothing new added.
In the last fortnight or so I've lost 8 fish. No visible symptoms or marks on them. Basically one by one they stop eating, become lethargic, fins clamped, seem breathless and die within 24 hours. One developed a small ulcer under the eye.
I thought it was perhaps vibriosis so treated with furazolidone but have since lost more in the same way.
Can anyone suggest what this may be? Im fairly sure it's internal bacteria but I do not know how best to treat next. Any help would be greatly appreciated!

 

[coralbandit]

I would try Kanamycin and Nitrofurazone.
In the US they are sold under names like Seachem Kanaplex,and API Furan2.
I had a similar(so similar it is scary) scenario years ago with my breeding group of swordtails .
http://www.myaquariumclub.com/columnaris-and-what-i-have-learned...-1689.html
Strain 1
Good luck.

 

[wazzie]

Thank you, I'll get both and try! Lost another fish today already, it's getting a bit desperate!