I am curious since I have never had any trouble with guppies. What kind of water do you have? The reason I ask is that I know most livebearers, including guppies, do best with water that is fairly high in minerals. If you have fallen into the trap of trying to give your fish very pure water, you may be killing them with kindness. Nearly pure water has almost no mineral content but may have almost any pH. The guppies will do best with somewhat hard water and as Fish48 said, with a neutral to a bit high pH.
I have seen posts in other areas of the forum where people were trying to do right by their fish and were buying bottled water for their fish. If you are one of those people, stop doing that with guppies. Guppies will do far better with nasty hard tap water than with the nice soft water that many egg layers prefer.