fish flakes are an excellent and common supplementary food source for many invertebrates, including isopods, millipedes, crickets, and some snails. They are nutrient-dense, providing essential protein, vitamins, and minerals that support growth, vitality, and proper molting. 
 
Benefits
High Protein Content: Fish flakes offer a convenient, protein-rich supplement essential for growth and reproduction, which can be lacking in a purely leaf-litter diet.
Essential Nutrients: They typically contain a wide array of vitamins, minerals, and carotenoids (which can enhance the color of some isopods).
Readily Accepted: Most detritivorous invertebrates eagerly consume fish flakes, often quickly, which helps prevent food from molding in the enclosure.
Convenient and Long-lasting: Flakes have a long shelf life and are easy to portion and store without mess.
 
How to Feed
Supplement, Not Staple: Fish flakes should be used as a protein supplement to a primary diet of leaf litter, decaying wood, and vegetables. The main food source for detritivores should always be the natural substrate.
Small Portions: Feed only an amount that the invertebrates can consume within 24 hours to prevent the flakes from molding or attracting pests like mites.
Frequency: Offer flakes a few times a week as part of a varied diet.
Preparation: You can sprinkle the flakes directly onto the substrate or moisten them slightly, which can make them more palatable for some species like millipedes. 
 
Considerations
Check Ingredients: Ensure the fish flakes do not contain high concentrations of copper sulfate, as this can be toxic to invertebrates.
Variety is Key: While a great supplement, fish flakes alone do not constitute a complete diet. Continue to provide a variety of other foods like fresh fruits/vegetables (sparingly), calcium sources (like cuttlebone), and leaf litter for a balanced diet.
Feeder Insects: Fish flakes are also a common and effective food for "gut-loading" feeder insects like crickets and roaches before they are fed to other pets.