Too much current in my tank?

I just got a new tank, and got 4 tiger barbs and some sort of tetra, (i forgot the name) but the Guy at Petco told me they were good for new tanks. I put them in, (after i tested my water of course) and they are close to the bottom, and have to move their fins to stay in one place. Whats causing this? i have an Aqueon power filter 20, and its it fairly small, and the water water falls into the tank, about an inch from the top of the tank, and a bubble, that should not have a problem with that stuff. Can anyone help me? should i make the water level higher so that it doesnt fall from such a hieght? please help