Water chemistry
The common and Endler guppy are both naturally found in hard, alkaline freshwater environments. They do not do well in soft and acidic water, but beyond that they are supremely adaptable. The typical municipal water around pH 7.5 and 20 degrees GH is just about perfect for guppies of both kinds. They can be also kept in brackish water, and make a good addition to the low salinity brackish water aquarium containing species such as gobies or killifish.
The swamp guppy is more particular about its living conditions, and seems to need slightly brackish (at least SG 1.003) to do well. When making brackish water, always use proper marine salt mix, not "tonic salt".