I purchased some northfin jumbo 4mm on their Australian website today so hopefully that will arrive soon and he will enjoy it. Maybe I worry myself more then need beYes, the 4mm sinks. If they still hold off, try presoaking the pellets in the juice of frozen bloodworms. That should change their mind, then slowly wean them off the BW juice, until they are back on 100% dry pellets. Good luck.