To develop your Fish you need to get him in School…

In the Level 0, your Fish needs to get a specific experience value to evolve, but at the level 1 they need more things.

They have to go to school to know with item they need to develop for next level. To know the item, you have to pay 250 Aurux , unless if your Fish is a professor. In that case he will just pay 100 Aurux .