. . . For instance, I want to maintain a tank volume of
234000gal. I have a water demand that varies from 1400-2400gal/min (gpm),
with an average of 1600gpm. The supply flow rate can range from
1200-1800gpm. . . .

Starting in C2:
=IF($E$1-(A1-B2)=B2,MIN($E$2,$E$1-(A1-B2)),B2)

Where E1 is your max volume of 234000. And E2 is the max inflow of
1800.