As a friend who keeps a log would say
Guppy1-Snakeskin - Male
Neon Tetra 1 - Male
Neon Tetra 2 - Female
Black Neon Tetra 1 - Male
Technically they all have names. They all have history. He just doesn't do things like give them human names, as they'd never know their own name and a name that identifies them is more useful.
If Neon Tetra 1 passed away he might replace it
Neon Tetra 3 - Male
If he knows the parents, that will be in his logs as well. He's got logs all the way back to 1984 when he started as a teen. Several of those fish are from the original bloodlines or at least 2nd or 3rd cousins to those from the original bloodline.