That was pretty much spot on about the Goldfish. There was a trigger for when it reached a certain size to stop growing until it gets moved to a larger area but I had used "<=" when I should have used just "<" in the If statement so it was triggering continuously. Simple enough to fix!
Your suggestions on the audio and visual cues for things happening are helpful. I'm sure that will take some experimentation when I get to it.
I imagine the core of the game like one of those merge games, where you merge two or three things to make one larger/better thing and repeat. So it takes 40 pieces of food to turn a Minnow into a Goldfish, Tetra, or Danio to start. To evolve them further they would need perhaps twice as much food or a smaller amount of a higher quality or larger food. (I haven't built that particular difference into the demo quite yet. All evolutions take 40 foods right now.) Once you unlock a predatory fish, you may need to feed it 40 minnows, or 40 of the larger fish to get it to the next stage. Other than feeding them more, it may simply become more complicated what you have to feed them. Maybe they need a specific combination. Maybe they need to be in a specific biome to evolve. Maybe they have to perform some other action first.
You may need a large population of some of the lower stage fish to evolve the higher ones. This is where the ecosystem management comes in. You'll want to keep the lower populations large enough to keep the larger fish fed. The player may unlock some sort of auto-feeder as they get further, to automate the lower fish's evolution so they can focus on the higher fish.
And that goes into the ultimate goal of the game, which is to keep evolving the fish, discover all of the species and fill all of the biomes. So my focus at the moment is making that clear at the beginning of the game (needs some work obviously) and making it engaging and rewarding early on.