Betta

I wouldn't attempt it. Their tales are quite fancy which could cause the fish to be even more interested in them.
 
Depends on the temperament of the RTS, but probably not a good mix, as a 20 long doesn't give the betta much room to hide if the RTS gets aggressive.

Do you already have the RTS or betta, and are trying to find suitable tankmates?
 
seems like the 20g long would be too small for the RTS anyway. not a matter of how big they get, but how much they move around.
 
Hmm.... kinda varying answers. To fill you guys in I got a 20 long that has no fish, just some small snails, and a lot of plants and driftwood. I want to put a fish in and those are my 2 favorite other than a pleco. I am just tossing ideas around really. I don't want many fish just a few larger ones that will be able to live in a 20 long so by large i don't really mean large? haha make any sense. I have other tanks that I can move them into if they get to large but would rather have one or 2 inhabitants in the 20gallon long just to help the plants out that are not too boring or unattractive. Suggestions would be great.
 
RTS will get too big for a 20 gallon. Maybe a gourami? However, gouramis are NOT compatible with bettas, so keep that in mind.
 
If you're just looking to provide movement in a planted tank, maybe go with 3 or so giant danios and a handful of cories. The giant danios are always moving (as long as the tank isn't overgrown with plants) and are a somewhat attractive, hardy fish.
 
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