Killifish

The Killifish (メダカ, Medaka) is a tiny fish found in the holding pond between April and August at any time of the day. It is the only "fish" found in holding ponds that is truly a fish.

In Animal Crossing
"Hoo hoo! Hullo, little fishy! Aren't we a tiny little fellow! Adorable! Why, I believe it stuck it's tongue out at me! Hoo-rumph! Nevertheless, we'll take care of the little blighter! I'll keep an eye on this mischief-maker myself!"

In Wild World
As with all fish, the killifish may be given to Blathers as an addition to the museum's aquarium.
 * "I heard that killifish recipes are common all around the world. Sadly, I've seen nary a one, not that I've searched... I haven't a clue as to their taste, even. It IS a fish, though, I can say that!"

The killifish can be found in the first aquarium tank after being donated.

In City Folk
Upon donating to the Museum in, Blathers will say the following:
 * "...Hoo, here's a morsel of interest--killifish are often used in observational studies... This is because they are translucent, wot? Indeed, their innards can be seen clearly from without! In earlier times, folks used killifish to discover how a body processed food. Well, suffice to say, the killifish is a marvelous little wonder that gives people a true eyeful!"

It can be found in the left tube tank.

In New Leaf
Upon donation to the museum, the information board in front of the tank which houses the killifish reads the following: "Killifish are small, travel in schools, and are known for having eyes higher up on their faces than others. They swim in shallow areas in rivers and mostly eat bugs that float on the surface of the water. This is why they have mouths as high up their faces as their eyes, making them effective hunters. Being the same temperature as the water they inhabit, getting grabbed by warm hands can shock them."

Further information
It is unclear what sort of killifish appears in. Judging by the size estimated within the built-in encyclopedia, it could be a Moonlight Medaka or "Japanese Rice Fish". However, all killifish have common traits, noticeably, their translucent skin which has made them an interest in biology due to the visibility of their digestive system. They are a harmless, common, aquarium fish.