How Sivatherium has said, you have chosen the worst day to visit the park. The price increase depending on the demand the aquarium expect for that day, normally is higher on holidays. I think in a normal day the ticket could cost like 26€ or something like that. Hope you enjoy the rest of the city. I also recommend Bioparc (the other zoo of the city) before Oceanografic because of the price and because it has normally less people but in this week is going to be crowded also

