Wishiwashi @ Choice Band  
Ability: Schooling  
EVs: 252 HP / 252 Atk / 4 SpD  
Brave Nature  
IVs: 0 Spe  
- Liquidation  
- Earthquake  
- Flip Turn  
- Toxic
you could try this as like a trick room breaker. You might need wishpass support though, as like mentioned, its not good after becoming smol fish
Wishiwashi @ Leftovers  
Ability: Schooling  
EVs: 252 HP / 252 Def / 4 SpD  
Bold Nature  
IVs: 0 Atk  
- Whirlpool  
- Protect  
- Toxic  
- Scald 
Trapper, protect for stall moves and lefties recovery
Wishiwashi @ Chesto Berry  
Ability: Schooling  
- Rest  
- Liquidation  
- Earthquake  
- Flip Turn
rest for recovering back to school form.
Wishiwashi @ Assault Vest/ Choice Specs
Ability: Schooling  
EVs: 252 HP / 4 Atk / 252 SpA  
Quiet Nature  
- Scald  
- Hydro Pump  
- Ice Beam  
- Flip Turn
AV/specs
Wishiwashi @ Figy Berry  
Ability: Schooling  
EVs: 252 HP / 4 Atk / 252 SpA  
Quiet Nature  
- Flip Turn  
- Hydro Pump  
- Ice Beam  
- Earthquake
Pinch berry to keep it out of smol form one time, as after dropping to 1/4, it eats the berry rather than reverting to smol fish. 
Basically all the sets that could possibly work that I could think of
but again, its not very good in general, and probably not worth raising one for the battle tower
also it has a different role as jellicent, as jellicent is more bulky, but I think its still worse