Top fin filter 20g

[Mark_the_shark]

I've seen my male guppies with large tails get a surprise when they swim right up next to the inlet. They escape just fine though.

Oh I see. How about bettas?

 

[OnTheFly]

Bettas, or any other healthy fish will be fine with a filter that size. Won't harm a fancy male guppy either. Just startles them if they slap a inch tall tail against the inlet.