Killifish

The Killifish is a tiny fish found in the holding pond between the months of April and August at any time of the day. It is the only "fish" found in holding ponds that is truly a fish.

Donating in City Folk
Upon donating to the Museum in, Blathers will say;

"...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.

Encyclopedia Information
Once the player has caught a particular fish or bug, they can find information about it in the bug or fish menus. Once the killifish has been caught, the following information can be found in the fish menu:

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.