Thanks to those that responded to my questions.

I have just completed adding salt at the 1gm per litre rate (over 2 days).
Also slowly raising kh up to around 80ml/l
I also have another tank (2ft) with some harlequins (7), 1 bristlenose, 2
swordtails. This tank also has never seen salt or had kh raised.. Just
checked it and it was 20ml/litre so am increasing kh and adding salt as
well. This may have attributed to the deaths of 3 guppies and 2
swordtails......Which never showed any obvious signs of illness. I have
been quite ignorant of the value of a good kh level. I always new it was low
but just assumed that as I could control ph with ph up or down and that it
wasn't worth worrying about. It seems that maybe the swings in ph may have
stressed the fish. Knowing this I must say that for a begginer I can't
recommend Harlequins high enough as they have never shown any signs of
problems even through my ignorance. My bristlenose has also been a trooper.

Thanks Elaine for telling me about Methylene blue. I have changed 30% of
water today and intend on changing it again in a couple of days (adding
corresponding amount of salt of course). I think I will go to my local FS
and get another med with acriflavin and malachite green and no meth blue as
I feel another dose maybe neccesary as 1 corey still shows signs of a fungus
type infection although it does seem to be getting better. As for the female
fighter she is missing a lot of scales but seems to be hanging on ok at the
moment.

My plants have suffered an amount of die back probably due to the meth blue
which I have added in the past.. This may have contributed to the ph fall
due to the rotting plant material.

If you have managed to read this far then sorry for rambling and thanks
again.