I agree that the fish sprites have a lot of visual charm and would work well in a final product. Combining fishing minigames with tower defense is a great idea! I like that objects like weeds and rocks can be used to buff a fish and that catching fish grants XP that can be used to upgrade their abilities.
It's kinda disappointing though that most of your catches grant 0 XP. I understand that you intend to implement fishing upgrades, but it feels better from a player perspective to get even just 1 XP per fish with a vanilla fishing rod to buy an upgrade that costs 2 XP rather than catching 2 fish to gain 1 XP to buy an upgrade that costs 1 XP. It's the same amount of work, but getting at least 1 XP from each catch is a more rewarding player experience (at least imo haha).
That's only a suggestion though. This is a really cool prototype with a lot of unique elements and it'll be cool to see where you go with this. :)