Carbon dioxide from the air dissolves into seawater and becomes carbonic acid, which sheds a proton to make bicarbonate (HCO₃⁻) and drop the pH. But the ocean is a buffer: those freed protons are immediately mopped up by carbonate ions (CO₃²⁻), converting them to still more bicarbonate.
So the buffer softens the pH swing — at the price of the very carbonate ions corals, pteropods and shellfish need to build aragonite and calcite. Add CO₂ and you gain bicarbonate but spend carbonate.
What actually dooms a shell is the aragonite saturation state, Ω = [Ca²⁺][CO₃²⁻] / Kₛₛ. When Ω drops below 1 the water is corrosive — shells dissolve faster than they form — even while the pH is still above 8. Cold, deep and upwelling waters cross that line first, which is why Southern Ocean pteropods are already pitting.
The simulation stopped unexpectedly — the lesson continues without it. You can move on; nothing you did was wrong.