I had my carnivore tank set up yesterday and was feeding them chicken nuggets. They returned 110 coins plus one pearl. Most of them were rare or uncommon (Isosceles Squid, Ribbon Seal, Baby Harp Seal, Caribbean Reef Squid, European Squid, Excited Golden Lionfish, Red Lionfish, and a Piranha) and they all flashed happily when fed -- and then hid offscreen. I'd like some smaller carnivores but am not sure the common ones will drop pearls as often as the uncommons and rares do.
I haven't counted to see if they all drop the same number of coins.
Today's tank is the (Legendary) Night Sky Angler, Baby Harp Seal, Candy Angler (apparently all Anglers eat chicken nuggets), Isosceles Squid, Caribbean Reef Squid, European Squid, Golden Lionfish, Ribbon Seal, and a Piranha. We'll see what they drop.
Many of the fish don't seem to want a specific food. They'll be happy with basic foods -- I haven't compared gold drop rates yet.
I tried fishing with the Rainbow Pellets, but am not sure they were any better than the Glowing Red Pellets.