Guppies are a Primary fish and are easily converted to saltwater. I have 2 males and 7 females in a 20 gallon breeder with 4 sailfin mollies. The salinity is 1.021 solid. They were converted over a 7 day period. I began acclimatization in a salinity level of 1.012 and slowly salinated the water to the current level. If you do not believe it is possible, call my LFS and check with Dean at Slice of Nature in Vandalia, IL. He is who turned me on to the fact of saltwate guppies and mollies. PM me and I will give you his number. By the way many guppies in the wild are naturally brackish as are many primary fish. Check out the book "Aquarium fishes of the world" by Ivan Petrovicky page 286, which states that "Guppies are found in brackish as well as salt water on or near the islands of Martinique and St. Thomas." (Carribbean) Same book page 280 concerning mollies, Note made "If the salinity is gradually increased the fish are capable of living as well as breeding in seawater with a density of 1.024 to 1.028 which corresponds to the environment of the coral sea." I am neither ill informed nor stating falsehoods. I am an Aquarist and only want to promote the hobby. My mollies and guppies readily breed in this level of salinity, 1.021. I was incorrect that all puffers are brackish as the Carinotetraodon family are fresh only (Somphong's puffer) while most others are brackish (tetraodon family). Sorry for the mistake concerning the puffer.According to the same book, page 478, it states that "most species of puffer live in tropical and subtropical seas and only a very few dwell in fresh water." quotes used from the above mentioned book.
