Basically, the bigger fish don't eat the smaller food items, and thus leave a mess in the tank. Also, fish like larger or medium-sized puffers will not be able to get all of the food, and sometimes the food sinks before they can get it.
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10 Gallon: 3 Bleeding Heart Tetras, 5 Zebra Danios, 1 Dwarf Puffer, 1 Pleco, 9 Neon Tetras, 2 Corys, 2 Catfish, 1 Kuhli Loach
20 Gallon: 3 Zebra Danios, 1 Golden Puffer (Auriglobus Silus)
55 Gallon: A lot of little fish
100 Gallon: 1 Banded Leporinus, 2 Tinfoil Barbs, 5 Clown Loaches, 3 Bala Sharks, 1 Bichir, 2 Cichlids, 2 Gouramis, 1 Fire Eel, 1 Pleco