Through years of selective breeding( ie. inbreeding ) the guppy is actually a very fragile fish. The slightest difference in water chemistry is enough to do it. Todats guppies have weakened immune systems, more genetic flaws and are overall less hardy. Thats why so many people have a hard time selling guppy fry, you can match water conditions almost perfectly and still lose a whole batch. A few years back my neighbor gave me some guppies ( excact same water) and within two weeks .... no guppies. If you are really interested in guppies I suggest endlers livebearer, look a lot like a wild guppy but with more intense colors and are more hardy