Here are the ZAA minimum standards for Nile hippo care and housing
"1. Nile
a. For one animal, a paddock enclosing 1,500 square feet, 5 feet high, with a pool of water, 120 square feet, 5 feet deep, equipped with a ramp or steps. For each additional animal, increase paddock and pool surface area by 50 percent of original area.
b. Access to a shelter shall be provided."
From:
ZAA Standards
Granted the Dallas Zoo may have different standards to follow, but this shows that your measurement is not far off from the ZAAs' minimum.
Just curious, is this hippo pool part of your plans for the "Wilds of Africa" exhibit. Don't know if you guys are planning a physical renovation of the exhibits, but I have a suggestion. If possible, you can have the monorail or whatever train system of your choosing and have it go right through the hippo pool, like a flume ride, and let guests be immersed in the hippo's world.