IMO an aquaclear 20 on a 2.5 gallon with a betta is just outright ugly. Bettas do not like much current at all. With the long finnage it makes swimming difficult on them. My bettas that are in 2.5s usually have sponge filters or the whisper 3i which is an internal filter but its air driven instead of motorized. Instead of using the carbon pack it comes with, I cut a piece of filter sponge to slide into it in place of the carbon.
Many Killifish can go well in a 2.5 gallon tank... but you must keep it covered if you keep them. They also love planted tanks.
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