It is believed that the life cyle of monos is as follows: babies are born in freshwater where they stay until they are juvies which is when they move to brackish water. They stay there for a large amount of time and move out to sea when they are adults, to return to freshwater to breed then again return to the ocean. This is why Monos are so adaptive (and no it doesn't matter whether it is a Sebae or Argenteus). Monos should ideally live in brackish when young then get adjusted to salt as adults. I have 3 ~5" Mono sebaes in full saltwater. They love it and are happier than ever. So if you don't have a high brackish tank/saltwater it is best to avoid these. I have seen adult Mono sebaes in freshwater but I use the term adult loosely. They were old enough to be adult but were only about the size of my guys now, maybe smaller. Mine are still juvies, I got them ~6 months ago when they were 1" juvies. Anyway, hope this helps and good luck!