You know, this is a quaint little game - and I'd say you're on the right track to deliver a great message with this game! Let's go over the positives first:
1) Neat art here! It's not the best, not as cohesive, but it works to sell what you're trying to tell and it's good enough to look at. Worth commending! The sound also blends in well enough but a bit of ambient music such as bubbles, or waves will sell the mood so much more. Definitely recommend that.
2) Great storytelling with games as a medium. I personally like your score system - it helps guide the player to identify the food stuff and avoid junk - and also invariably sell the message of the infinite junk via the idea of seeing your score plummet to negative infinity. You've done good here - I like!
Some other qualms that I have with the game are:
1) The controls are very, very wonky. 'Jumping' feels very floaty and left and right has a lot of drag to it which makes it feel like you're sliding on ice. I think you could benefit just from having the fish accelerate to max speed faster when you're moving left and right - and make your 'jumping' accelerate faster on initial spacebar. I think you were going for the similar swim function found in mario games - which can work, you just need to implement it better really.
But honestly, great game. I think you did good here. And for something done in 3 hours, this is quite impressive, being able to fit in all the story as well as the art, and gameplay. Nice!