I have been doing a project to create hardy saltwater guppies to then give out to my LFS to see if they can sell them. It starts by breeding guppies in low end brackish water, then raising the salinity by 0.001 specific gravity with every generation until they reach full marine then continue to breed them in full marine for a year. Finally I will select only short fin guppies for this process, so that they don't have any issues swimming.