Before we go any further, I'm going to say the inevitable: don't buy a fish until you've researched it. Assume the clerks in the pet store are at about the same level as clerks in grocery stores. (Lots of pet store clerks are very knowledgeable, but the difficult bit is figuring out which ones, so to keep things simple, assume they know nothing!)
Polypterus senegalus is a very popular, easily maintained fish and has featured in most aquarium books for the last 20 years. Getting reliable information isn't difficult. It is basically a peaceful insect-eater, and hunts by smell, so takes most any small invertebrate foods. It will eat things like neons, but otherwise isn't predatory and shouldn't be kept with aggressive, nippy, or indeed predatory fish. I've seen these poor fish literally nipped to death by African cichlids for example.
I'm assuming the salinity is low, less than SG 1.005 (= 9 g/l, or 1.2 oz/US gal). Now, over the next few hours, do a series of 20-25% water changes, replacing the brackish water with plain vanilla dechlorinated tap water (not from a domestic water softener though!). By the fourth such water change, the salinity should be so far reduced that the bichir will settle right down. If it's not, just keep doing water changes until he settles. Space each water change by 30-60 minutes. Simple as that!
Cheers, Neale
oh well that is crap the people at petsmart told me they were brackish water do you have an idea on how to get rid of the salt then
Aqua Boy