Thanks for the advice. I'll see what I can do next time I get a bit of cash. I'm flat broke at the moment because I've just bought a second filter for the tank and a load of conditioner, food, meds etc.

I don't think its a fungal infection though. My largest fish is recovering from an external ulcer at the moment and wouldn't a fungal infection in the tank attack the wound? I'm still learning at the moment but he's getting better by the day, so I think its just limescale. Are there any water additives to deal with this problem?