Guppies.
They are either fucking awesome or just plain tough.
I started an aquarium few years ago.
I bought few fishes:
4 danios, 1 corydoras, something I don't remember and 4 guppies.
Soon some of danios died and guppies started breeding.
It was (and still is) extremely "casual" aquarium (no knowledge about fishes or plants).
I bought few fishes more. Corydoras died, new danios died too, some algae eating fishes (those guys with sucker instead of mouth) died, but guppies still replicated.
At any given moment there are at least 20 guppies in my small (40-60 liters) aquarium, which is more than sum of all other fishes I ever had and through entire history of my fishkind only about 7 guppies died.
I gave about 40 of them to other people because they overpopulate like fuck. My aquarium sucks - dirty water green with algae, glass is covered in green strands of living plant shit, ground has bigger % of shit than of rocks and sand... and yet those fuckers thrive.
Seriously, what the fuck is wrong/right with them?
Pic partially related - mine are red and look less decorative and more like wild fishes.