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By breeding a endler and a common guppy you ruin the pure strain of endlers, therefore producing hybrids. It's not advised and most shops will not take them and they will ruin the strain. Endlers are endangered in the wild so most people avoid doing it.
You guys are assuming that they are starting out with a pure endler strain to begin with, which is not likely considering it's being called an 'endler guppy'. So if a person is to take on the moral effort to continue/multiply a particular species then it'd be smart to start with known pure bloodlines.