Translation:I have a big family, so I should buy a lot of water.
Sorry, but I don't agree with this at all. By the way, Pol didn't either in the comment above.
"I should be buying" could show urgency (I should be doing it right now) or some kind of contrast (I should be buying a lot of water, but I'm not doing it). But here, "I should buy" is the better choice, no matter if we're just talking about one day or making a general statement.
Your answer was made the main answer about a week ago. But I see the report that almost certainly is yours and it's green, there's nothing wrong in it. Either it was a bug or the process of applying the changes made on our side is slower than usual. But I do see the changed main answer at the top of this discussion page.