Sorry for leaving it so long to reply, I have left things as they are to see if there are any developments. The adult fish have pretty much stopped flicking now - I'm not at home during the day but in the evenings I sometimes don't see them flick at all. The juvenile fry, however, are flicking. It varies - sometimes they do it a few times an hour, sometimes not at all.
One thing that has become more obvious is the slight curvature of the spine of the two remaining female adult guppies. The best way to describe it is that it looks like the tail and head is weighed down, to create a gentle curve. Also, one of the smaller fry is bent a bit like a boomerang shape, but the other way so it's tail and head are higher than it's middle. Also the gravid spots sometimes appear a bit pinkish.
Finally one of the adult females has developed two small white spots on her body. Last night I gave her a salt dip but this morning the white spots are still there. As one of my other females died very rapidly after developing a white, flaky "saddle" this is concerning me a bit.
Finally, and I'm describing pretty much everything odd here but one of my female guppies has a dark "bruise" colouring on the top of her head, between her eyes as you look down. It looks a bit like a gravid spot in her head! Is this something to worry about?
Oh yeah, and I found a dead fry this morning. I felt over the last few weeks that I had turned a but of a corner with this but I feel at a loss again
My thoughts:
Is the flicking a bit of a distraction and not related to what is killing my fish?
Something seems to be slowly picking off my fish one by one - the female Guppies seem particularly susceptible - could this be to the stress of birthing?
NTD - the more I read online the more some of the syptoms seem to fit, but I think that everytime I read a disease description! The fish don't really appear lumpy before the die, though. How quickly does this normall spread? Whatever it is is not raging through my tank quickly, but slowly picking off the fish...
Columnaris - one of my deaths really seemed like this was the problem - white flaky saddle and rapid death seems to fit almost exactly what I have read online about this but so far but I would have expected more casualties more quickly if this was the case (although I did treat the second tank where this fish was with an antibiotic).
Immune systems - is there something wrong with the immunde systems of my fish that is making them susceptible to diseases that are normally present in the environment and that most fish can fight off?
Gill flukes - the adults seem to have stopped flicking so this is seeming less likely but it is still in the back of my mind as the initial symptoms pointed me in this direction.
TB - The fish don't seem to be losing scales so I discounted this but some of the symptoms fit. Obviously as I'm often putting my hands in thr tanks, this is something I want to discount.
Finally, is there anywhere that will perform an autopsy on something as small as a guppy? This might sound odd but the uncertainty is starting to stress me out!