There are lots but it would be easier to ask what kind NOT to put with him actually.
No fish that can fit in his mouth and that includes full grown size.
And fish that are known nippers like triggers, especially a tank that small.
Can I ask what type of lion you were considering? A volitan is going to be too large for this tank very quickly. Are you thinking of a dwarf lion? You have more options if you go that route because of the size difference.