Salinity levels

I've never seen one. In my experience the preferred SG of a brackish fish depends on what species and who you ask. Is there any specific fish you had in mind?

I don't know much about brackish tanks but here is a link that might help



http://badmanstropicalfish.com/brackish/brackish.html

In the wild, as ive heard salinity can fluctuate so as G. said, it depends on the fish you intend on keeping.

I would ask the store what they keep them at, generally I go off of that. IMO I would aim for 1.004-6, but the store may keep them at fresh, and if so, you must transition them over slowly.

1-welcome to brackishland. they're a great group of fish, and lots of hardy choices - my black-fin silver (columbian) shark is 10 years old, and i've had monos and scats last for years.
2-badmans is an excellent site.
3-as usual, the lfs is always going to be the problem. the fish are usually kept in fresh, which is not terrible if they're very young, because that's where they usually start as fry. if you're halfway lucky, they'll keep them with african cichlids, which at least are kept at a high pH, but regular salinity. if you ask them if that's a brackish tank, they'll probably say yes, even tho it's not. it's very rare for brackish fish to be kept in an actual brackish tank (high pH AND enough salt to get the SG up) in your lfs. although i'm not a puffer keeper, there are plenty of posts here about how miserable they do at the lfs (especially walmart) because they're kept in the wrong type of water. that's just the way it is, unfortunately.
4-but not to worry. monos and scats are especially indestructible, and are fine for live fish cycling of a new tank. (i know of a pro who uses monos to cycle his marine tanks.) the difference here is that whereas usually you prefer not to buy a fish that just came in that's getting over the shock of a long trip, with brackish sometimes the longer they keep the fish in the store (in the wrong water-type), the worse off they get. if you can get them outta there and into some proper water, they'll probably perk up.
5-try starting off with a SG of 1.002-1.005, like phoenix said. float the bag a little while for the temp to equalize, then pour a half cup of tank water into the bag every 20-30 minutes or so, until the bag is mostly tank water, then net them and pop 'em in. this should acclimate them just fine. remember, in an estuary, the salinity goes up and down with each tide, so they're designed to tolerate all the ups and downs. the real risk of wild sg swings is killing all your good bacteria that you've worked so hard to cultivate.
6-one plus of all of this is that the SG change will probably kill any bad germs that came along for the ride from the lfs.
7-i've never kept archers, even tho i like them. from what i've read, they can be problematic. they get big, aggressive, usually need a school, and feeding them might be trouble. i've read of someone who had an archer-specific, mangrove tank, with the water at half height, with room up above for crickets to hang out in the dry portion so the archers could knock them down with their "spit".
8-now i have a school of 5 monos in my 75g. used to have a mixed school of silver, green and red scats. pretty easy. as they get older/bigger, i ratchet up the sg occaisionally at water changes. now i'm at 1.012, and that's as high as i'll probably go. monos especially can tolerate full marine, but i don't see the need to go that hi.
9-i also have a 10g QT with a much lower sg - 1.004 or so. that gives me more wiggle room with new additions.

hope this helps. good luck
steve

what kind of setup do you have?