I take it this is either a very new tank or that you dont do weekly water changes?
Your fish is dying of nitrite poisoning and there are 2 things you can do.
1-add salt 1/2-1 tsp per gallon... you said you already did this. Hopefully you didnt add either regular table salt, or more then 1 tsp per gallon, but this will help negate the effects of the nitrite in the blood stream.
2-This is actually the most important. You need to change out 100% of the water though not right away, you should do it in 20% chagnes every day for a week this will get rid of all the impurities in the water...If you do too much too fast you will kill him just as fast as if you leave him in the water just like it is. At that point do 25-30% changes every week to keep all the levels down.
Hope that helps and that your fish pulls through let us know how it goes for you.
------------------
Adam
Come see my website! Adam@cichlidfish.com Got ?'s about HITH?
Read this to learn more!