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Mollies and Saltwater (and some other stuff)


Here's something I put together in 2008, hope it gives some idea about keeping Mollies in the aquarium. If you want more info about the salt there are a lot more pieces of information on the web now, but it's a starting point for you...... :)

There is some confliction about the safe keeping of mollies as a freshwater fish.

Hopefully this will help people to understand, in layman’s term, the physical attributes and adjustment to their environment.

Mollies are livebearers, with the scientific names of poecilia sphenops, p. velifera and p.latipinna, and are found throughout Central America. Many of their habitats include river estuaries, although they are found in lakes, rivers, and in some cases coastal waters.

Because of their environmental spread, they need to be able (unlike most freshwater fish) to cope with different water parameters. In all cases, however the water is hard and alkaline. The differences are within salinity of the water. Their genetic make-up is to be able to cope with these differentials, with the idea that, should it be necessary, they can survive in an environment that they have not been raised in. For instance, if there is a disease seemingly prevalent, then they can move to a more saline area to prevent the spread and kill off parasites and such.

When keeping such fish in an aquarium environment it is usual to mix them with other fish, most of which they will never come in contact with in the wild, and most of which would not be able to deal with fluctuating water changes.

It can be noted that, when kept on their own, they can survive in total freshwater, as long as it is hard and alkaline if the fish are hardy to begin with. On the addition of different fish, with different immune systems, and therefore carrying different pathogens, it can often be seen that a problem will occur, usually with skin-based things like fungus, whitespot, velvet etc.

When these mollies are then moved to a more saline environment, then they tend to improve and live normal lives.

For them to live in a total marine environment is relatively rare, but it has been noted that black mollies (p. sphenops) do very well in a marine environment, against the other true (and hybrid) types which will normally expect a change between fresh and brackish water on a day-to-day basis.

This is why it is recommended that mollies are kept in a slightly brackish environment, as they appear to handle any fluctuations better, presumably because the pathogens that they are prone to are not so virulent in saltwater.

Mollies should not be kept with other poecilia species, such as guppies and endlers, as they can, and often do interbreed, which makes for some less hardy hybrids.

Mollies, as other livebearers, are an interesting fish for the aquarium, but their requirements must be understood before taking the plunge and adding them, especially to a full community tank.

Some mollies like adult sailfins, can reach 8", although most average 4"-6", so at least a 3' tank should be provided.


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