It’s awfully weak and notably tasty, so everyone is always out to get it. As it happens, anyone trying to bully it receives a painful lesson.
– Pokémon Moon Pokédex
Wishiwashi is a strange and unique Pokemon. It doesn’t evolve, however, starting at level 20, it can form change into the School Form, which gains a huge boost to its stats. It loses the School Form when it’s below 25% HP though.
In Solo Form, it has the lowest base stat total of all Pokemon at only 175, 5 lower than Sunkern (the previous record holder).
In School Form, it has higher base stat total than the pseudo legendary Pokemon (eg Tyranitar, Garchomp etc) at 620 (still lower than Slaking though). However, its speed is really slow (only 30), and its HP is the same as in Solo form at 45. So even though School Form has good base stats, it’s not as competitively viable as it looks just going by stats. However, because it has good stats, it’s pretty good for a run of the main game.