Brittle nails (onychoschizia) come from repeated wetting and drying, harsh soaps, nail polish remover, low humidity, aging, or occasionally iron or thyroid issues. Most cases are environmental, not internal.
Moisturize nails and cuticles, wear gloves for wet work, limit acetone remover, and keep nails trimmed. Biotin has modest evidence for brittle nails. Nail Exodus provides antioxidant herbal support as part of an overall approach.