Lots of good info in here, so I’m going to toss in mine. Based on your posts it sounds like you’re just running in circles. If it were me I would call the LFS or a friend and see if they have any seeded material (ie sponge or ceramic media) or mulm. I would put that in the tank and give it a few days. To me it sounds like you’re screwing up the cycle by trying to correct it. Bacteria, like snails only populate to the demand (food supply). If the amount of fish you have don’t produce a lot of ammonia you won’t have a lot of bacteria. Adding new livestock just creates an imbalance.
Could be totally wrong, just my thoughts.
lrbaquatics.com sells mulm.
Don’t buy any more fish, you’re just wasting money.