Ok. After much Thesaurusing, I think I've come up with a reasonable progression. It's Water based, rather than Fish based.
1. Rain Drop.
2. Puddle Splash.
3. Pond Ripple.
4. Creek Trickle.
5. Stream Eddy.
6. River Torrent.
7. Lake Swell.
I'll go and set them in...
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Angels, Black Mollies, Bristlenose', Clown Loaches, Cory Aeneus, Gibby, Guppies, Hockey-stick Tetras, Keyhole Cichlids, Otocinclus.