Hi musicman,
You are in a Fish-In cycling situation and the number one goal in that situation is to try and save the fish. Your water changes are not large enough to be accomplishing this. The idea of smaller percentage water changes is all about protecting fish from small shocks when everything else is normal. When they are swimming in poison, as yours are, that trumps the concern about shock and is a much more urgent priority. The ideal action to be taking during fish-in cycling, when you see a number like NO2=1.6ppm, is to use good water changing technique and change large amounts of water. I would start with 70% and dose the conditioner at 1.5x or 2x (an additional plus would be to use a really trustworthy conditioner like Seachem Prime or Amquel+ that will do an especially good job of converting any ammonia to ammonium) but not more than 2x, and also roughly temperature match (your hand is good enough for this but don't be too slap-dash about it.) Or you could do two 40% ones like this with an hour inbetween if you have that kind of time available. You can do any number of these with the hour in between and keep testing to see if you're getting the nitrite(NO2) close to zero ppm.
Once you're past this stubborn part of getting the initial nitrite(NO2) out of there, you can settle down to the normal fish-in process. The goal is to be a detective and figure out what pattern of percentages and frequency of water changes will keep both ammonia and nitrite(NO2) below 0.25ppm before you can get back for another water change. The problem with nitrite is that is attaches just like oxygen to the fish blood hemoglobin protein and destroys it, turning the red cell into brown mush. This causes suffocation and the first symptom is nerve and brain damage, except that since its a fish, you don't usually see the symptom and to you it just seems the fish died really young at some later point. Now its true that different species show different tolerances to these poisons, but many of the estimates of these tolerances that are passed on come from fish farm operations and situations where the judgements were not about the subtle things you care about as an aquarist, but more crude things like 10% die-offs and such I believe.
Anyway, am assuming you have one of the good liquid-reagent based test kits and know how to use it, so all of this should make pretty good sense.
~~waterdrop~~
