Wait and catch fish by clicking and getting it to the green. Return to surface button is where you go for upgrades.
New update! Added a new frenzy that occurs every once in a while which makes fish spawn 3x faster and gives 1.2x more money then usual when you catch a fish. Sadly removed the rarity upgrade :( It just was to overpowered and it wasn't really fun once you bought it a lot. Also 6 quests so far :P