ok - that may well be beyond me, its kinds wierd, calling expert folks! In the meantime let me see what I can think of...
To have nitrates of 100ppm you must have a fairly large bacteria colony so that rules out a tank that hasn't started cycling yet...And you say water changes aren't helping, but the tap water is clean
My first thought given the latter fact is are you sure your test kit is working? Other than that lets go back to basics...
What size is your tank? What fish are in it? How often do you do a water change? How much water do you change when you do this? Do you use a dechlorinator when you change the water? Have you recently washed out/changed the filter sponges? Did you wash them in tap water? What filter do you have? Do you have an ammonia reading in the tank? Have you recently messed up/changed the gravel in your tank? I just noticed you said 'even with chemicals' - which ones did you try?
My second theory (hence all the questions) is that the tank is seriously overloaded for this point in its cycle and/or you aren't changing enough water to keep on top of things...but those figures are very, very high...I'm suprised you aren't seeing side effects...
Sorry for the endless list of questions - its complicated to diagnose at a distance
To answer your original question btw I wouldn't advise adding any fish till you see a 0 nitrite reading at least 3 or 4 days in a row...
For now, given you say the tap water is ok, I would start doing some serious water changes (2 times 25-30% a day, for example) and see if the numbers start to come down...
HTH
aj xx