A small barb in one of my tanks seems to have some kind of rash (a bit like white spot) on its tail fin. It appears to be the same colour as the tail (orange) only a bit darker. I can't see any more of it on any other part of the fish or on any other of the fish in the tank. The fish seems to be behaving and feeding normally, though maybe flicking a bit. Does any one have any idea what this might be and how best to deal with it!?
Cheers.

If you provide all of the tank inhabitants it would help with deciding a treatment. Fish illness are difficult to nail down even with a picture so any further description will help. Are you capable of isolating the fish for treatment or would this have to be a tank wide deal?

Only other fish are a Texas cichlid and a rainbow fish. They are all fairly small, the biggest is the Texas which is about 4 inches (100 mm). The tank is quite big, an 'L' shape about the equivalent of a 5 footer. Unfortunately there is no treatment tank available. The rash sort of looks like grains of sand.

My first suggestion would be to try aquarium/kosher salt 1 teaspoon per 5 gallons added over a day rather than all at once, premix in a cup of tank water to dissolve.

See if that improves things after a couple days.