Thanks for all the replies, as i am just getting into the hobbie and already have the ac 30 from previous use, about 10 years ago, ill just stay small and if i get more fish ill keep in mind that ill also have to be getting more filter.
Thanks again
I have an AC 30 on my 10 gallon, so for a 20 I'd go with the 50 or 70. Even if the fish are being stressed by the current, you can always adjust it. Remember, its always better to have too much filtration than not enough.