This answer is entirely dependent on the species in question here, so here’s a list of species and the diets we’ve observed them enjoy!
All species of isopods tend to enjoy leaf litter, snake shed, fruits, vegetables, algae wafers, rotted wood, moss, freeze dried minnows, fish flakes, plants, and fungi grown from bioactive substrate.
Depending on the species of isopods, they will enjoy a portion of this selection more than the rest:
Armadillidium klugii: a diet high in protein, moderate in calcium
Armadillidium maculatum: a diet high in protein, high in calcium
Armadillidium vulgare: a diet moderate in protein, moderate in calcium
Porcellio dilatatus: currently observing, but noticed immediate interest in algae wafers
Porcellio laevis: a diet high in protein, moderate in calcium
Porcellio lamellatus: currently observing
Porcellio scaber: a diet high in protein, high in calcium
Porcellionides pruinosus: a diet high of protein, moderate in calcium
Cubaris sp.: a diet moderate in protein, high in calcium
Trichoniscus sp.: a diet moderate in protein, high in calcium
Trichorhina tomentosa: a diet moderate in protein, high in calcium
Cylisticus convexus: a diet high in protein, high in calcium