Is this too powerful for a 20 gallon? I'm thinking about switching my filter over to this, but I'm not really sure if it's meant for larger aquariums.

I know that the more filtration, the better but I don't want too strong of a water flow coming from the back of the tank and making my fish not want to swim near the filter, or stressing them out. It's just hard to tell what a new filter will be like without hearing recommendations and others' experience first.

I think it's rated for aquariums between 20-50 gallons? So yes you should be fine with this one. I'm sure I've heard of many people using it for a 20 gallon.

You can adjust the flow rate if it's too strong