This would be for the tall Jellyfish and Cuddlefish type of fish that are of the size Small Fry. You'll see the icon on the right of the fish name that tells you it is a Small Fry.
To expand on the Professor's answer. All fish have 2 descriptors. One is type, the other size. This adds up to 9 possible tanks. Tall, Regular, and Large for type and Small Fry, Keeper and Whopper for size. The tank must match both.
