Tetra's With Black Lumps

Under7

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Tank size: 120l
pH: 7.6 (measured with a TetraTest 5 in 1 strip, as are the below)
ammonia: no test
nitrite: 0
nitrate: 25
kH: 10
gH: >10
Tank temp: 26-28 degrees
Volume and Frequency of water changes: 30l weekly
Chemical Additives or Media in your tank: Melafix and Cycle on water change (with Nutrafin Aqua Plus to treat the tap water), Api Aquarium Salt irregularly (usually when a fish looks peaky)
Tank inhabitants: X-Ray Tetras, Leopard Danios, Harlequins, Rosy Tetras, Bronze Corydora, Upside Down Catfish
Recent additions to your tank (living or decoration): Nothing for over a year
Exposure to chemicals: None
Digital photo (include if possible): Pretty much identical to http://www.reptileforums.co.uk/forums/fish-keeping/814771-sick-phantom-tetra-cannot-diagnose.html
Tank Age (How long the tank has been up and running): 4 years, moved to new house 15 months ago
Recent Events in the tank: Nothing
The type and size of your filtration: Fluval 3 Plus
Location: UK

I think I've got some kind of endemic disease in the tank, and despite looking it up on the net and in all the books I've got I still can't identify it. To date it's killed 6 Phantom Tetra's, 1 X-Ray Tetra and 3 Rosy Tetra's over about 4 years, and now one of the two remaining Rosy's is showing the early signs. I've tried treating it with high doses of aquarium salt, Melafix and Interpet's anti-fungus, general tonic and anti-internal bacteria, all with no success. Using Melafix seems to extend life expectancy for a week or so, but any fish that's shown symptoms has died within weeks. It generally shows in one fish at a time (once two Phantoms had it), often with months in between - it's been nearly 12 months since I last saw it.

The main symptom is a large black/dark grey lump that grows around the base of the tail fin. In a couple of cases similar but smaller lumps have grown around the dorsal fin (as is the case now). If the fish survives for a couple of weeks a small white dot appears in the centre of the black lump, steadily protruding outwards until I presume it bursts. The phantom that lived the longest had most of its tail drop off shortly before it died. The Phantom's and the Rosy's seems most susceptible, I've kept Glowlights, Panchax and some other fish in the same tank previously and none of them caught it. Apart from this the only disease in the tank has been a single outbreak of white spot, and the fish appear otherwise healthy and happy (the Upside Down cat is nearly 20 years old, the cory about 3 and the rest 2-3).
 
Finally found the instructions for the liquid Nutrafin water quality tests:
pH: 7.5
Ammonia: less than 0.6
NO3 Nitrate: between 10 and 20
NO2 Nitrite: 0
 
The salt is really bad for the catfish and cory, first off. The symptoms described sound a bit like an internal parasite. Can you post photos?

EDIT: Did a bit of research, and it looks like the photos I saw of peduncle disease, which is a bacterial infection. Dosing with a wide-spectrum antibacterial will likely cure the disease, but will also kill the bacteria in your filter, and you'll have to start a fish-in cycle after the treatment is done.
 
I only ever do a half-dose of salt as I'd heard it was bad for cory's in high concentration, should I avoid even that?

As for an anti-bacterial, Melafix and Interpet's anti-bacterial don't seem to work at the recommended concentration, is there something else you'd recommend (I've been trying to get hold of some Methylene Blue but it's often out of stock) or should I try upping the dosage?
 
I avoid salt entirely. Melafix isn't a medication. It's just an herb, melaluca. I don't know about the Interpret brand. You should be looking for an actual antibiotic medication. The one mentioned most here is Myxazin. Methylene Blue is for fungal infections, whereas Myxazin is an antibiotic. Where are you located?
 
UK, Hampshire. Haven't come across Myxazin in the shops but it seems to be easy enough to order off the net, I'll get some and move the Rosy to the quarantine tank for treatment. Thanks for the tips.
 

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