Anything that wont nip its fins, they will also sometimes eat smaller tetras when the tetras come to the surface to feed. i've had 2 ABF's, make sure you have a really good lid, my last one suddenly disappeared one night and i found a tiny hole at the back of the lid where the back piece of glass had slid forwards somehow
I never found him at all, i think my dog ate him, he was full size too
they can jump VERY far, i have found mine on my bed about 1.5m away when i left my lid open and came back with a bucket while doing water changes. also they need somewhere to hide, the best thing i found was those plastic plants you stick to the side glass with heaps of leaves for reptiles, just stick it to the side with all the leaves down in the water and they hide in it, floating plants like water sprite are good too.
my water is 6.7 pH, 4dGH and they seem to love it. you may have to re arange your filter outlets as they dont like to much surface agitation, if you can get them to eat flake, etc then thats great but ive never had any luck and have to feed a small cricket or 2 a day, they are big eaters but wont eat anything thats not floating. also dont worry too much if he doesnt eat for a few days once hes in your tank, it may take some time before he feels comfortable. anyway hope i have been of some help