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I have kept and bred guppies for years, but recently all the new guppies that I have purchased have died within 3 days together with all of my other guppies.
the other fish in the tank are all fine.
I have recently started using a co2 diffuser which has lowered the ph to 6.4 .
Can guppies live in water with a low ph?
has anyone had the same problem?

What did your ph start at? Guppies can survive most any normal ph level but if your adding lots of co2 and lowering the ph alot there could be 2 possible problems. You could be dissolving too much co2 into the water and suffocating the fish or the ph may be changing too quickly. Especially when using diy co2 mix with yeast and sugar you can end up with a fluctuating ph if your not careful and while fish can survive a broad ph range they cannot survive quick ph changes.

It also could have nothing to do with the co2. Have you quarantined the new guppies and did they show any sign of illness? Guppies frequently carry diseases that could have wiped out all your other guppies. If your other fish are fine it's probably more likely that an illness is the cause of death since co2 would have an affect on all the fish. Although some fish like gouramis have evolved to survive water with low oxygen or high co2 better than others.