What exactly do you mean by "predator fish", it's a term that is used somewhat loosely. A 40g is a tough tank for a true "predator fish", which I would list as lions, scorps, anglers, toadfish, groupers. A 40g would be too small for most of these fish, and the ones you could house in a 40g, would likely eat any other tank mates that could live in a 40g. Many true predatot fish also have specialized care, especially in regards to feeding. So more research on your part may find you more answers.