There are many fish that have appeared throughout Happy Tree Friends, and they usually make only one appearance and a minor one at that. However, some play a pivotal role in the episode in which they appear in.
An electric eel appears in Off the Hook, in which it electrocutes Russell while he is being dragged along by Lumpy's fishing line. The same eel can be seen getting regurgitated by the whale in Get Whale Soon. In Sea What I Found, Russell is seen using an electric eel as a welding torch.
Great White Sharks
- Main article: The Sharks
Great white sharks appear in the episodes Happy Trails Pt. 2: Jumping the Shark, Wipe Out!, and Wingin' It. A shark also makes an appearance in Petunia's Summer Smoochie and in the games Going Overboard and Strandead.
A hammerhead shark is seen in Sea What I Found, in which Russell uses it as an actual hammer to build his machine. The shark later attacks him after the machine falls apart.
Manta rays appear a few times during Sea What I Found. One swims behind Russell as he approaches the shipwreck. Then a small school of them can be seen when Lifty and Shifty drive Disco Bear's submarine towards the undersea volcano.
- Main article: Russell's Piranha
A school of piranhas are first seen in Water You Wading For when they attack Cuddles in a lake. These piranhas are purple and don't resemble Russell's Piranha. Russell's Piranha first appears in Something Fishy, in which Russell presents it to his class during show-and-tell. The piranha soon escapes thanks to Giggles, killing her and Petunia, and injuring Flaky, before eating Russell's head to the bone. A bunch of piranhas are later shown at the end of YouTube Copyright School.
In Whose Line Is It Anyway?, Russell swallows a live pufferfish, which inflates in his throat and chokes him.
A sawfish is seen in Sea What I Found, swimming near the shipwreck. Russell later grabs the sawfish and uses its rostrum to cut a hole through a sunken ship. It later attacks Russell after the machine he built falls apart.
A seahorse briefly appears in the beginning of Off the Hook when Russell swims to the ocean floor to eat a clam.
A swordfish appears at the end of Whose Line Is It Anyway?, impaling Russell through his head as he reels it in with his fishing rod.
Generic fish appear various times in the series, as wild animals, characters' pets, or food.