i reckon do a slight water change, keeping all fish buying to a minimum for now, and add something to re spike the ammonia ( i used fish food) and restart the cycle, and if you must have something in there, try something small like a snail and that can help in continuing the cycle and help aid in beginning the load. also you might want to try getting some API stress zyme, it basically contains the bacteria that grows in the tank to remove ammonia, and nitrites.
also do you have a protein skimmer? they add a head of dissolved oxygen to the water and also aid in removing proteins which lead to the water going bad.
what was the SG
of your tank when you placed in your fish? and what Ph?