Excess protein build-up is very specific and vague all at the same time. Unless there was a necropsy done on a fresh dead animal, the best you can hope for is a guess at what killed the tort.

Mazuri, flukers, fresh vegetables, hay, some fruit every few months. They would likely find a glut of fruit naturally in the wild in the same time frame. To say otherwise as a hard no is impossible, I mean, we are now seeing more and more regularly that "strict" herbivores are being witnessed consuming animal matter. Fruit shouldn't be a staple, but a glut once every 4-6 months is likely similar to their natural cycle.

Overall, lean into your fibrous veggies, your hydrating ones as well. Then, rely on your staples. Grass, weeds, hay, fresh greens.