ah, ok good old henderson-hasselbach and acid dissociation constants. always messed me up, will continue to mess me up
low pH -> lots of H+ -> drives equation right so predominant species is ammonium, neutralizing its toxicity. In higher pH situations (less H+), the predominant species is ammonia, which is toxic.
so bottom line: ammonia is toxic, ammonium is less toxic. If you have low enough pH, more of the ammonia will convert to ammonium, rendering it less lethal to fish. However, because it's an equilibrium constant, there will always be some ammonia present in its nonionized form, so it's not like low pH is the panacea.
Thanks
@essjay