I can't add to this discussion, except to say that sometimes leaving things alone is better. As one of you has mentioned in the last couple of exchanges, fiddling with KH has side effects, such as increasing TDS and raising the pH. Taken in context, both of these do and will affect the fish in this tank, being wild-caught soft water species, and we do know scientifically that increasing TDS shortens the lifespan of these fish. So while the ammonium may be present--or may not, frankly, since we can't scientifically determine if various factors might be affecting the test--it is not having a detrimental effect on the fish. In my view, the fish are better off as things stand than they will be with any sort of fiddling with KH.
Your observations on aragonite are spot on. I tried this, only as half a cup added to the canister filter, and within a day the pH rose from 6.6 up to 7.8, while the GH and KH remained near-zero. This was an experiment to raise GH marginally that failed. Back when I had tap water coming out below pH 6, I used 2 or 3 tablespoons of dolomite in the filter, same tank, and the pH remained steady at 6.4 for years until the dolomite gave out. I tried the aragonite, but at the same time the water authority raised the pH to 7 or 7.2 so that played into the equation as well [this lowers to around 6.4 to 6.6 in the established tanks]. But a sharp rise in pH is a detriment of this method.