The quick response is to do a partial water change to lower nitrites. It is possible that you added too many fish at once for your bacteria to handle. Exact fish counts and weight plus tank volume will get you a more precise plan of action.
how much growbed do you have? high nitrites are not good for the fish.. what's your ph? are you adding aeration?
the cycling process usually results in a nitrite spike after the ammonia spike, sounds like you were a bit short of actually being cycled.. as noted, water change will help lower the nitrites, i'd salt to help the fish deal with it...1-2ppt (i usually go 3ppt but with catfish, i'd go lower)
no.. you can use pool salt, or (as i do) use water softener salt - table salt contains anti-caking agents that may be harmful to fish.. kosher salt will work in a pinch but it's kind of expensive
a refractometer will measure salinity...
Whatever you do, don't feed them anything at all!!!!!!!!
Feeding will end up creating more ammonia in the system as the waste breaks down and thus more nitrites. Fish can go weeks without food so don't worry about them starving
Here is a good visual representation to understand the nitrogen cycle
also pick up one of these:
to filter out the chlorine
were the gills brown, or bright red? high nitrites cause "brown blood disease" in fish..
did you salt at all? this does help, but is not a guaranteed "cure"..