This is the anime adaptation of the mermaid encounter found in test 3 of the manga.
It follows exactly the same plot - Tsukune wants to join the swim club and the president is an evil mermaid who wants him.
It's gorgeous to see this in colour - we get her green hair and orange tail which is a nice reversal of Ariel. And of course their personalities are polar opposite as well.
The anime also makes a little more of the other mermaid members of the team.
So our evil mermaid attacks Tsukune:
Moka dives in to rescue him and the girls fight. Moka turns into her super vamp alter-ego and trounces the mermaid in a mid air fight. Again, really loving the use of colour - the pool become red like blood.
Moka is victorious and we get this nice scene, not seen in the manga, of the defeated mermaid caught in a net.
Short and sweet. Evil mermaid monster overcome, not much more to say really - except we never see her legs in the anime, she's always in the water.