Depends on what kind of turtle you are getting. A mud/musk turtle will generally stay on the bottom so as long as you have different height platforms like driftwood or rocks to climb to the surface then the waterfall current shouldnt bother him. If you are getting some kind of slider who prefers to swim then the current will be too much for a baby to swim through in a 20 gallon. If it were me i would get a 40 gallon breeder which can be bought from petco during there $1 a gallon sales and build an above tank basking area so your turtle has more swimming space. Below is a picture of my old 40 gallon breeder with a emperor 400 filter on the side.
Ok I have 2 baby eastern painted turtles
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