This is a really difficult question which can only be answered by science and stadistics!
So let's begin.

Many researches have been done to search fishes and what paths they picked. Thankfully, I organized a list of the most important fishes.

• Siluriformes (or Catfishes)
  The Siluriformes have been proved to mostly go UP. They don't mostly go in group, so they fear that going down would force them to be with every other fish that goes down, which as we have seen in our private experience, is a lot of fishes.
  
  
• Pygocentrus nattereri (or Piranha)
  The Pygocentrus nattereri usually go with their childs and family, protecting them from outsiders. They think that going DOWN is the right way to go, due to how much more easier it is for parents to protect their small children.
  
  
• Acanthocybium solandri (or Wahoo)
  The Acanthocybium solandri is one of those fishes whose answer is hard to understand. This fish lacks the ability to regulate it's body temperature, so most often than not they will go UP, for how much more colder and easier to live in it is, in contrast to DOWN. However, sometimes they will choose to go DOWN as a way to dodge the predators that roam UP, even if the temperature is much hotter.
  
  
• Archosargus probatocephalus (or Sheepshead)
  The Archosargus probatocephalus are more liklely to go UP than most other fishes. The food they consume, crustaceans, usually live UP. I have nothing more to say.
  
  
• Phobrinosaulus congurus (or Krendy's Pinky)
  The Phobrinosaulus congurus is a small fish that most often than not goes alone to their quests. Messing with them 'causes them to, essentially, go wild and attempt to either scare or straight kill the enemy. Their nature 'causes them not to care about which path to go, so some may go UP, while others DOWN.
  
  
• Hypophthalmichthys molitrix (or Silver carp)
  The Hypophthalmichthys molitrix is a fish devoted to going DOWN. They will do anything and everything in their power to make everyone who they care about go DOWN, even manipulating them.

After the research, one clear answer has been shown to this dilema: The choice is not one to be taken slightly.
We can't point at an exact answer, not UP nor DOWN, due to how complex this question is, but whoever who hasn't went astray from their path will find out if their choice was the correct one by the end.