As long as you feed it, it will be ok and of course as long as you can keep the water parameter stable, for salinity, PH etc...
I have one in my 75 gallons with small fish and it never bother anything. It is not aggressive as it went to place itself right beside my crocea clam and it is toutching it but the clam has been standing this for a month now without much problem. It sometime retract its mantle a little when the anemone toutch it but more often it does not react much to it.
It does move around if it is not happy when you put it.
I would say go for it in a 20 gallons if you can keep it stable.
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