Jim, sort of scratching my head as to why you’re getting carbonate deposits with “normal” levels of alc and calc. If you’re also maintaining magnesium, you shouldn’t be getting all this carbonate deposits. Assuming all things are normal, I’m guessing the microenvironment where the alc/calc is added (your sump) experience abnormally high concentrations until diluted….which end up precipitating out.
However, I did like you Alka Seltzer analogy. Good stuff!
As far as something a little stronger than vinegar, check out muriatic acid….it’s actually diluted hydrochloric acid (HCl). It will make your carbonate fizz like crazy. Just handle with care. You can find this in the “concrete” area of HD or at a pool store.
Yeah, I don't completely understand it. The hardest deposits seem to grow in areas of high flow, like near the pump intake. Some of the deposits are soft and crumbly and seem to contain precipitated strong calcium chloride, if you break them the pH shoots way up. Some of these formed fantastical shapes in the current, like teardrops. I did have to learn to keep the drip lines well-separated in the sump. Muriatic acid seemed like the next pitch to me too. It scares the hell out of me, and I was't sure if it was safe to use in the tank, like does it get hot?