Both mollies and guppies require similar envionments for optimal health.
Considered an easy fish to maintain, both mollies and guppies are desired by first-time aquarium owners as well as seasoned fish owners. Both are live breeders, meaning offspring or "fry" are birthed directly into the water, swimming immediately, rather than laid as a fish egg to emerge later. While they do not do well in the same tank -- mollies can be aggressive and nip the long fins of guppies -- their habitat requirements are strikingly similar.
Salt
Both mollies and guppies can survive in freshwater tanks, although it is well-known that mollies like brackish water -- a mix of fresh and salt water -- for optimum health. However, both species are found in the wild in brackish water. A high saltwater content can shorten the lifespan of guppies; it is important to use a very weak solution -- such as 1/2 to 1 tablespoon of salt to 10 gallons of water -- to provide a natural habitat for guppies, whereas mollies can tolerate a higher salt content. You can purchase marine aquarium salt in pet and fish shops. The benefits of using brackish water also include an increased resistance to disease. Both fish benefit from having 25 percent of the tank's water removed every one to two weeks; adjust the salt content in the new water accordingly.
Water pH
Both mollies and guppies prefer a harder water -- one with dissolved minerals, that is not softened through the use of chemicals that leave only sodium behind. The pH level of that water -- which shows how much acidity or alkalinity is present -- should be tested to ensure it is between the range of 6.8 to 8; guppies prefer a 7.8 level for breeding. Neutral water measures 7; below that, it is more acidic. Higher levels indicate it is more alkaline. Sea water is over 8, and moving fish too far away from their natural habitat -- such as the brackish water for mollies and guppies -- can cause stress and allow disease. You can perform testing at home with test kits, test strips or even handheld meters, available for purchase at fish and pet stores.
Plants
While both species can survive quite nicely in an aquarium that is devoid of plants -- plastic or real -- fish do like to have hiding places inside their habitat. Live plants discourage algae growth, and because they mimic the natural habitat of guppies and mollies, they can keep your fish healthier and less stressed. For breeding, both species prefer to have plants in the aquarium for their young to hide in.
Temperature
Both mollies and guppies can handle a temperature range of about 10 degrees, from 70 to 80 degrees Fahrenheit. However, an ideal temperature for both is around 76 degrees, with a fluctuation of a few degrees in either direction. Although both species are considered a "tropical" fish -- found in warmer parts of the world -- the water should not go over 82 degrees to protect the health of the fish.
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