Let us start over, let us stick to respectful conversation, even when we do not agree.
I have some history with being angry and it does not foster good communications.
My apologies as well for the Scottish vote references, I am a resident of Jersey, the Channel Islands, so your Queen is my Duke (go figure) and of course Jersey is a Royal Peculiar, which is I, gather, the reason they let me in.
I suggest you read my Lowering KH via Lime & Muriatic Acid
thread. That may explain part of it.
Avoiding getting technical, I need to ask you to accept that this is an “equilibrium reaction,” a fancy way of saying this reaction happens in both directions.
In your case this means that even though you added enough acid, to overcome the buffer. KH, carbonate/bicarbonate hardness, at normal freshwater fish (especially planted tanks) pH this is pretty much all going to be bicarbonate, is principally a buffer against pH changes.
Any acid (sulfuric or otherwise) you add will “use up” (actually it is changing some bonds) buffering capacity. When you add enough acid to overcome the buffering the pH drops until a new “equilibrium” is reached. If you leave it alone at this point and nothing is done to remove the bicarbonates, the process will begin to reverse and those bicarbonate bonds will reestablish.
Once you have added enough acid to “use up” enough of the bicarbonates, to make the pH permanent, aerate (vigorously) the solution (aquarium in this case) to drive off the CO2, so that the carbonates/bicarbonates cannot reform.
I will look into this API pH Down, somewhere somehow they are modifying the buffering arrangement. I do this all of the time with phosphates, which as it happens also provide plant food.
I suspect the API thing is to avoid using phosphates due to the (irrational?) fear many seem to have of phosphates.