I'm very glad that my comments came across positively!
Without knowing the structure of your code, my first thought as to why you sometimes get teenage mutant ninja goldfish growing out of control is maybe an interruption in a change in said goldfish. Like if you have the scale of the fish change at some point and then the player somehow either interacts with that fish or triggers a state change for the fish, then the scaling process is interrupted and instead of getting the trigger to stop, it just keeps going. Your guess is way better than mine though.
Here's a bit more on feedback:
I like the idea of particle effects for eating. Depending on how many fish can end up eating at once, a sound for eating may be irritating to the player unless you limit it (if I feed 20 fish and then hear the same soundeffect go off rapidly for 10 seconds it could sound like audio artifact. I don't mean to discourage sounds from eating, I'm just not sure what the right balance would be.
As for evolution, a gradual change in color/size might be too subtle for players to feel like they accomplished things. Unless you did a slight gradual change to show the player its progress toward a new evolution, then give it a finale with something like a morph animation from its current size and color to the new one.
Giving the fish the ability to change their environment would be visually appealing to the player. For example, if when swimming by flora or objects, those things could move or react slightly. Like if a fish swam through some falling food, the food would change its path slightly because of the movement of the fish.
Since this scene is so close to surface, some god rays that get interrupted by slightly moving lilipads could enhance the scene.
With feeding/growing/evolving is the core mechanic, I think it would be cool to have different ways to feed. I have enjoyed my fair share of clicker games, but clicking for a 1:1 work:reward ratio forever will get dull. Maybe being able to hold down to spawn a bundle of feed. Or click and drag to spawn a line. Maybe add a food shaker (like a salt shaker). I think your current direction with growing/evolving is good - feeding different combinations of food to see what the fish evolve into will keep engagement for a time.
What I am wondering about is the player's goal. What are they striving for? What will be the loop that gets tied to your core mechanic? Maybe you said so in your video or in the story intro and I missed it.