For my cycle, I dosed enough to bring ammonia up to 3 ppm and then waited until it dropped to 0, then dosed up to 3 ppm again. Once it could bring 3 ppm down to 0 in 24 hours I waited until my nitrites went down to 0 as well
Okay , feeling better about how the cycle was done. Done correctly , the major advantage of a fishless cycle is that it does build the large, full blown bacteria to meet a full load. I'm assuming that you did it right since you are not missing the major point. Some do miss the big points at times.
Moving on to the next idea? With fish dying, is there any chance of not spotting a dead fish back in a corner where it becomes a total ammonia factory?
Other thoughts might be something small being done to knock out too much of the bacteria. New tanks and filters tend to be somewhat easy to upset and that can also come when we are most worried and we try to hard to keep it all clean.
Cause may be hard to find at this point but what seems clear is that the bacteria was doing enough to convert the 3PPM of ammonia and that should have been enough but now it is not working so you have to fight what is, rather than what should have been.
I would do some things depending how easy or what is on hand. One I like to keep on the shelf for crisis, is a box of Ammo-carb or Ammo- chips. They are pretty cheap and easy to hang in a bag somewhere that water flows around them and they do take ammonia out of the water. They also have the advantage of lasting nearly forever if they are cleaned and restored. Soaked in salt water per directions on the box?
Water changes for sure will reduce pollution and if done so that the fish are not shocked with totally different water or cold, etc. it can do the full job but sometimes it is a lot of time and work.
Done clean the filter, stir the bottom or even wipe down the glass for a while so that you do nothing to kill as it is the vital point that is missing. If it was there at one point, it does bounce back much quicker.
Only some meds knock out the bacteria but Melafix is not one of them. Those meds can't tell the difference in the good bacteria and the items they are meant to kill.