When you say you changed the filter,did you change it completely? The Media [sponges] only needs rinsing in old tank water or you will kill off all of the good bacteria! That would explain the nitrite spike. Try doing 50% water changes every other day to let the filter catch up.
Anything bought from a store that claims to reduce anything can be substituted by water changes, just make sure you use a good de-chlorinator!
One last thing, if he is not going to the surface to breath, try dropping the water level so it is easier for him!
Lisa x