It will, but whether it should be there depends on whether you plan on cycling it or not. Since he is sick, change the water as often as needed to keep the ammonia below .25ppm. You can worry about cycling when your fish is well enough to handle it.
Aww bless him.... you definitely did! In all honesty, everything just sort of went to he|| in a handbasket for a while there with all of your fish falling ill. Well at least you're the wiser for it. Just focus on the other little guys now.