Good options: paper-based bedding (Back-2-Nature, Megazorb, cardboard or shredded paper), kiln-dried aspen, hemp bedding, and fleece liners over a solid base. A mix works well - fleece on shelves, loose substrate in the deep base for digging. Avoid: pine and cedar shavings (the aromatic oils are linked to respiratory and liver problems), scented or dust-heavy bedding, cat litter with clumping clay, and cotton-wool style nesting fluff, which can wrap around toes and tails. Depth matters. Ten centimetres or more of loose substrate in the base lets rats dig, which is a real behavioural need, not decoration. Litter training: rats usually pick one or two corners. Put a tray there with a distinctly different material, move a few soiled droppings into it to mark it, and clean it daily. Males scent-mark constantly regardless - that is marking, not a training failure. Cleaning rhythm: spot-clean the litter tray and any wet patches daily, change fleece every two to three days, and do a full substrate change weekly or fortnightly depending on group size. Leave one handful of old, clean nesting material behind so the cage still smells like home. If the room smells strongly of ammonia, you are cleaning too little or ventilating too little. If your rats are restless and re-marking everything, you may be cleaning too thoroughly, too often.