I don’t know if they use chloramines. I wonder if I could call and ask. When I dose my API tap water conditioner Inuse the dosage for chloramines if that makes a difference (5mL per 30 vs 1mL per 20 gal).Do you know if your municipal water supply uses chlorine or chloramine for a disinfectant?
Chloramine breaks down into ammonia when you use your dechlorinator product. What I'm getting at is, you are getting too much ammonia into that tank.
I know it's a pain, but I would do more like 80-90% water changes and would do another one or two and then measure.
Hopefully you have your tank in a spot where water changes are easy, or use a python water changer.
FWIW, once you run out of your dechlorinator product, get Seachem Prime, or Seachem Safe, the granular form. It's more expensive, but it's a better product and you use less of it per water change.
I use an Aqueon water changer. I think it is similar to the python, maybe not quite as good but it works. I will do another change in the morning.