Agreed that's low surface area, yet still some SA is in effect due to structures in the tank, corals themselves, the entire back wall coralline growth is very high surface area. Though that's not a proper display, it still eats ammonia fast fast
Those fish needed to be really big but they weren't for a 20 I'll bet
You weren't leaving food uneaten and littered about, that tank is treated better than that I can tell.
Adding bottle bac is not how we'd fix this even if it was a verified cycle issue, we'd add surface area.
Every surface there is already full with bacteria, there isn't less due to less fish
The surfaces in that tank have the same filter ability as if they came from a ten fish tank. Fish don't control the presence of our filter bacteria, surface area is the limiter along with flowpathing designs for the setup.
What did your ammonia testing say happened? Can't believe I'm asking that lol / denier of all non seneye test readouts / still curious to know.