What I like to do and works 99% of the time. Wait till about 3am when it's nice and dark and then turn on all the lights for the tank. It take about 15 min for fish to adjust to the change of lights, so they will randomly swim around. this is a great time to net/cup them out. Granted, Hippos may be a little different because they like to be in rocks, but hey, ya never know.