I think he asked you to check the pH/kH of the water both in your tank and in the shop from which you bought them and adjust your tank's pH/kH accordingly. Some of these breeders/sellers conditioned the guppies to their water quality rather than adjusting to known parameters for each type of fishes to lower the maintenance. Imagine if you have contrasting requirements, then you have to have separate filtration systems etc for each type of fishes. Since your information is lacking we cannot help you very much though e.g. established tank of what?? plants, other fishes, ornamentals.... etc.