Through this post (Why did my Oscar change color? ) I ask myself, how to identify an unknown fish?
What should I look at first? Size? Shape? Kind? Color? Biotope?
So my question is: what strategy should one follow, to identify an unknown fish?
Here are my two best techniques for identifying a fish:
Reading materials about the most common aquarium fishes. Websites or books often list the most common aquarium species with pictures. I have a book at home called "the bible of aquarium fishes" that has pages for many different tetras, cichlids, cories, catfishes, betta, guppy, platy, etc. Your fish is very likely listed there and you should recognize the picture.
Here's a website example: https://en.wikipedia.org/wiki/List_of_freshwater_aquarium_fish_species If you see a fish that looks a lot like yours, chances are you are at least looking at the right family of fishes. (example: catfishes are easy to recognize) Then you can check the detailed differences between the different species of that family to pin point exactly which one you have.
Taking a picture yourself and showing that (or showing the actual fish) to experienced aquarists that you know in real life, in a petshop, or online on a forum, imageposting board, facebook group, subreddit or any group or board dedicated to that hobby. If the picture is good someone will definitely recognize your fish. Make sure to ask more than one person to be sure.
Attributes you can use to differenciate your fish from others:
Once you identify your fish you can learn about its natural habitat and its preferences in feeding, tankmates, temperature, pH, does it tolerate salt, does it need salt, etc.