[TheOldSalt]

Biomax, Cycle, FritzZyme, StressZyme... they're all junk. I already told you the only two brands that actually work. Use one of those if you want results.

Simpte's question is a good one. Just what DID you use as an ammonia source?

I started to wonder if you had enough filtration on the tank, but after a review I can see that you do, so adding more won't help.

So, the ammonia was dropping and dropping so you kept adding and adding, but then it got too high before you stopped adding, and it kept dropping for a little while but then stopped.
I still say you probably added too much one day and either killed or mutated the bacteria.

In your case, if it were me, I would make sure the various filter media was not clogged and that water was flowing through it instead of around it. Then I'd add some Stability of BioSpira. After a week I'd change half the water. yes, half, and I do realize how big a tank it is. That's really all there is to do at this point except either wait and wait and wait in the hope it eventually corrects itself or just start all over again.
However, if you have mutated bacteria, then they'll just take over the filter media and limit the number and effectiveness of useful ones. To be extra sure I would change all the media in one filter and seed it heavily with good bacteria. A few weeks later I'd do another filter, and later on the third one. That should fix everything.

In the meantime, I'd go ahead and add some fish after that first big waterchange.