I use Biocatlet paper based cat litter in their hay tray.. but i change twice a day cos i am bit of a clean freak and they poop loads in it..... i use liners but in areas they wee more i use a smaller one on top so can just whip it out easy... the cat litter is £10.50 ( ouch) for 25lt from Pets@home...... not seen any other paper based cat litter anywhere x