My fish have skin infections - 2 have died
 

I have already lost 2 Red Line torpedo barbs in the last 24 hours. The first seemed to have no physical signs of disease, but today's loss had some kind of skin infection. http://picasaweb.google.co.uk/HolfPe...eat=directlink

One of my rainbow's has had a similar infection (looks like a scrape - so i thought at first he'd injured himself - still not sure). The rainbow doesn't seem to be getting any worse (or better) and has been like this for about a week.
Water perameters have been stable for ages - Ph 7, zero nitrite / ammonia, KH 5, Nitrate 10.
Weekly water changes. Using seachem ferts.
After yesterday's first death, i dosed 50% recommended measure of melifax (I have 3 gourami's in the tank) this morning - then within a couple of hours the 2nd Torpedo barb died. His picture is in the link above. Is melafix going to solve the problem, or do i need something else?
Appreciate any help I can get.