Is This Fin Rot - How To Treat?

[sws]

Hi
Recently bought 2 gold gouramis. the female had a small 'nck' in her tail which didn't seem to be a big issue. Ove the last 24hrs it's become much worse and very red around the tail base with a slightly white edge to the damaged fin. Photo is attached (hopefully). I've put in some melafix and primafix, is it fin rot and should i be using something else? All other fish are fine and the male gourami is in fine form.
All help appreciated - thanks
Shaun

 

[WILDER]

Size of tank in gallons or litres.
How many fish and which type.
Water stats in ammonia, nitrite, nitrate, and ph.

Red edging to the tail is septicemia and hard to cure once it advances.
She need issolating and a bacterial med.

Melafix is no good its an external med for cuts and wounds.

You need an internal bacteria med for septicemia.