TL;DR
Platy fish are exclusively freshwater fish that cannot survive in saltwater environments. Their physiology is adapted to freshwater, and even brackish water would cause severe osmotic stress.
In their natural habitat, Platy fish live in warm, slow-moving freshwater streams, canals, and pools.
They prefer heavily vegetated waters with mud or sand substrates.
Platies thrive in relatively stable water conditions between 70-82°F, neutral to slightly alkaline pH (7.0-8.0), and moderately hard water.
Adequate filtration and frequent water changes are required to maintain proper water quality.
Saltwater and freshwater ecosystems differ enormously in their chemistry, and fish physiology is highly adapted to specific environments.
Saltwater contains high levels of dissolved minerals and salts, predominantly sodium chloride. It has greater conductivity and alkalinity.
The ocean has relatively stable pH and calcium levels. By contrast, depending on location, freshwater habitats can vary widely in water chemistry.
Fish osmoregulation, the process of maintaining salt and water balance, is very different in saltwater versus freshwater species.
Fish that spend their entire lives in freshwater are highly adapted to compensate for passive water influx and salt diffusion out of their bodies.
They actively uptake salts through specialized chloride cells in the gills and excrete dilute urine.
Saltwater fish have the opposite challenge – they ingest lots of salts from the water but lose water to the environment by osmosis.
So they drink large amounts of seawater, absorb the salts, and actively excrete the excess through their kidneys.
Platy fish lack specialized adaptations for saltwater osmoregulation.
They suffer severe dehydration, electrolyte loss, kidney failure, and organ damage if placed in a marine or brackish environment.
Platies do not have the key ion-regulating chloride cells other euryhaline fish use to change their osmoregulation between fresh and saltwater.
When exposed to salinity, they experience negative impacts on blood properties, metabolism, and respiration.
Prolonged stress from improper osmoregulation will compromise their immunity and eventually prove fatal.
However, platies can tolerate slightly brackish conditions for short periods.
Brackish water contains modest levels of salts between fresh and marine water. Platies avoid brackish levels above 1.005 specific gravity.
While evolutionary adaptations allow certain fish species to thrive in saltwater or traverse between fresh and marine biomes, Platies lack such capabilities.
Livebearing fish like mollies or guppies are better equipped to handle brackish conditions due to their habitats near estuaries and beaches.
But Platy fish have evolved over millennia in inland freshwaters. Any attempt to keep them in a saltwater or reef aquarium would likely end in disaster for the fish.
That said, there are isolated reports of enthusiasts successfully acclimating Platy fish to low-end brackish conditions.
This involves an extremely gradual introduction over several weeks or months to water of 1.005-1.008 specific gravity.
Careful monitoring of fish behavior and health is required, along with optimal water quality and diet.
But long-term survival rates remain poor, so avoid exposure to salinity altogether.
For fishkeeping hobbyists looking to add color and activity to a saltwater setup, there are much better species options suited to marine environments, such as damselfish, clownfish, wrasses, and gobies.
The stunning colors and flowing fins of Platy fish are best appreciated in a freshwater tank with appropriate tank mates.
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