What is the most adaptable fish you can think of?

Mollies are indeed adaptable as you say, and the species Poecilia mexicana in my opinion is the most adaptable of them. This is one of the species that can adapt to live in environments with hydrogen sulfide, and in such cases, it will change the shape of it's mouth in order to better take advantage of the oxygen that's available at the surface of the water. Ones that are found in dark caves have adapted to that environment by having smaller eyes (no real need for sight) and heightened senses (allowing them to hunt and feed better in the darkness). When returned to a normal setting such as a open stream or a home aquarium, they've been known to lose their adaptations and return to a "normal" form. Other species of Molly are impressive as well, sulphuraria also lives in areas with high levels of hydrogen sulfide, latipinna and sphenops can adapt readily to live in either freshwater or saltwater.

If I had to give it to a group of fish though, Gobies get my vote. They're found in all sorts of environments, fresh, brack, salt, oceans, swamps, rivers, waterfalls, even land - and not simply surviving days on land, or crossing land to reach other water sources, but actually living on land. Of course, this is adaptation over many, many years, as opposed to the more "instantaneous" adaptation abilities of Mollies.

Mollies aren't bad, but my vote goes to the catfish.

While catfish thrive and enjoy clean, pristine water, if the water is too warm, too murky, or does not have enough oxygen to support other fish species, chances are you may find a member of the catfish family present. As a group, catfish are among the hardiest fishes living under some of the worst conditions. What other fish can survive for days and even weeks in a dried up pond by burying itself in the wet mud?

There is a difference between hardiness and adaptability. Adaptation is the ability to live in many different types of places not just stagnant water. One of the most adaptable fish i can think of is the convict cichlid. A hardy fish is the paradise fish.

bettas

they can survive in small bowls for years. they are fighters.

i remember 3 years ago, freshman year, my friend got a male betta and she kept him in a bowl literally the size of your two fists put together. i hadnt talked to her in a while, so a few weeks ago i asked her about her fish and i was like, hows Toby-James? (her fish).

and she was like: OH! i completely forgot about him! i havent fed him in like a week, i think hes still alive (she puts him at the top of the fridge) and she was like, yep i tapped on the bowl and he moved his head!

i think he must be dead by now, but if he was alive i would have taken him now that i know more about bettas.

For me glass catfish

chinese fighting fish