Poorly Guppies

Mornin' all

My guppies seem to have become afflicted with some sort of illness which is pretty much killing them off at around 1 guppy every 1-2 days  . They don't appear to have any obvious external signs of illness other than them having ragged tails (tails of the still living guppies look fine) and at present none of my other fish seem to be affected, including my other livebearers (Platies, Mollies).

I tested my water yesterday and the parameters look fine - 0 ammonia, 0 nitrites and approx 40 nitrates (my tank always seems to be around this level for nitrates). The only parameter which is perhaps out of their ideal range is pH which is generally around the 6.6-6.8 mark but it has been this way for as long as they've been in the tank (around 8 months).

Anyone have an idea as to what the problem could be??

Thanks

David

Hi Sue, thanks for the reply.

I haven't noticed any stringy poo or protrusions (but will keep an eye out) - they seem fine one day then the next they're floundering around on the bottom before passing away shortly afterwards.

I don't actually have a tester for hardness but our water company quotes our water hardness as being moderately hard (average 194.75mg/l calcium carbonate). Having said that, the 'out of the tap' pH is more like 7-7.5 so it obviously drops in the tank (a large piece of bogwood I have is possibly part responsible for this) - could the hardness also drop?

I think I've commented before that I've admittedly had more trouble with live-bearers than any of my other fish - but saying that, all my other fish were specifically selected for compatibility with my water conditions whereas I only really got the guppies/platies/mollies as a concession to the OH who liked their bright colours an fancy fins  (I've already told her than we won't be getting any more!).