F/W bacteria will transfer to S/W tank?

Well, the bacteria that process the ammonia and nitrites in FW haven't even been recently identified for certain. Marine Lands research resulted in the BioSpira product, and it does seem to work--but I think they indicate that there are different bacteria that process wastes in FW vrs SW. You can read some of theire papers here: http://www.marineland.com/science/nspira.html Most anecdotal information I've heard indicates the Bio Spira is effective in FW, if that helps.

As for how people can...Well, it's the web. Publisher bias is certainly present on their page--they encourage chemical treatment for most algaes. You may want to contact Reefscape and RTR--I think both have gone through the conversion process, and their advice might be useful.

In theory they are different bacteria, but the only valid research I know of is on FW from Tim Hovanec et al. I do not think this has been repeated for SW, unless we get another doctoral candidate with heavy funding and plenty of time.

I have taken FW tanks to high BW or SW (adjusting green spotted puffers from LFS sale in FW to their prefered SW) without any unoxidized metabolites (ammonia or nitrite) showing up in the water. But, these are fish that are spawned and grow in BW in the wild (as are scats and Monoes) and move to higher salinities as they mature. Because of that I have always made the adjustment very slowly, many, many weeks to months, with each step up in specific gravity being very small. I have assumed without proof (other than no upset to nitrification - puffers are very sensitive to such) that any switch-over in bacterial type doing the job is happening as slowly as the changes in density - as FW types decline the SW types develop. If they should happen to be the same bacteria, it would read the same on testing, so I cannot discriminate between the two scenarios by my practices.

If the water were changed rapidly from FW to SW I have no idea what would happen. No experience there.

HTH