You need to test for ammonia, nitrite and nitrate. The area and nitrite should always be zero. Once you know your tank cycles properly and your ammonia and nitrite are always there, it is your nitrate you then have to keep an eye on. Nitrate is nowhere near as toxic as ammonia and nitrite and fish can survive okay in even very high levels. However the higher the nitrate gets to poorer the water quality. To clear up the hole in the head you really don't want your might try getting any higher than 10 ppm. That's just my opinion, it's not based on any scientific information.
So what you want to be doing this test your nitrate on a regular basis for maybe the first week. I don't know if you told us how big your tank is, I have forgotten If you have. Obviously the smaller tank you have the quicker the nitrate levels will build. So you may well find yourself doing a water change every three days. It also depends on how many fish you've got to how quickly the nitrate will build. Good quality water conditions play a huge part in healing this disease.