In the wild, tortoises are grazers, they wander and eat constantly. Its hard to replicate that in captivity without having a large outdoor enclosure and making sure food is provided over the course of the day instead of all at once.
Most aquatic turtles on the other hand, they tend to eat and then spend most of their time basking in one spot. If they're swimming all the time, maybe their basking spot is not adequate? Of course, its completely normal for them to dive off their basking spot as soon as a human nears and then come over to the side of the tank and act like they're starving to death.
All turtles are very routine orientated, and if you stick to a schedule, you'll eventually find your turtle or tortoise waiting for you at feeding times.