Converting a freshwater fish to a saltwater fish?

No. The bodies of freshwater fish expel large quantities of water to retain salt to maintain a safe electrolyte balance. The bodies of saltwater fish do the exact opposite: they expel salts to keep from retaining too much water.

A saltwater fish in a freshwater environment would expel too much of the salt in its body and retain too much water. This would lead to several health problems, up to and inclding death. The opposite would hold true for freshwater fish in a saltwater environment.

There are fish that can be acclimated to either extreme for a limited period of time, but those species are not as common as 100% marine or 100% saltwater animals, and of those, very few are sold in the ornamental aquatics trade.

You could see if a petstore, try a Mom and Pop Petstore and see if they'll buy them, then you could get some tropical fresh water fish who'd love that, instead of putting a dog in the room, put a plant it'll live longer.

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