nano question

There are small, adjustable heaters, that will fit a 2.5 gallon aquarium, but will look very out of place due to their large size. I would recommend you get a pre-set heater (usually pre-set to 25.5 C or so).

As for fish, I would say maybe a single Betta, or perhaps a small school of Microrasbora.

Thank you for your kindness

Check out ember or ruby tetras. They are small fish that don't swim around that much so they might do OK in a small tank.

all fish will jump out of the tank if it has no top(bettas and schooling fish especially), and why clog the tank with a heater if you can place one of those terrarium heaters under the tank and conceal it well. set it to the right temp and youll have a nice setup thats free of that annoying heater look.

Assuming you will have an HOB filter, most small heaters can fit in a small HOB.

+1 pinboy.
and i'd say single beta *ducks thrown fish bowl*

be sure to get a lid if going with a betta.