Wow, this was a wholesome experience. A full game and fully polished. Great art, great audio, great fun. This is nothing you normally see for a game jam. :D I only have two negative points and one idea.
First negative point being that the balancing might be a little bit off and thus the game feels a little bit too easy, especially after you fully upgraded the attack speed. Second is that I only really seldom used the fish powers because they did not feel of much use to me (except the heal).
A nice idea could be to make the fish slots function as a sort of shield. So if an enemy swims into a fish slot circling around you, he gets damage and the fish slot is used.
Great work overall!