That's way too big of a filter for that small of a tank. No wonder the poor fish are getting thrown around!
I would second the opinion that you invest in a different filter. For a tank that small, you could actually do nothing more than a sponge filter, which has the advantage of providing filtering as well as a little oxygenation. If you decide to go that route, I would recommend either the Hydro Sponge I
or the Hydro Sponge II
. Both are rated for tanks up to 20 gallons, so it's just a matter of which of the two shapes you think would look better in your tank.
Or, if you wanted something a little more substantial, I would recommend you buy the absolute smallest size AquaClear HOB
(hang-on-back) filter, which would be the AquaClear 20
. Of all the various brands of HOB
filters, many of us have found AquaClear to make the highest quality ones. That being said, you will probably still have to do something to mitigate the current; just temporarily I have an AquaClear 20 on a 10 gallon tank I am working on cycling, and it creates quite a bit of current. I am currently using this nifty trick
to help break up the current somewhat, and it helps for sure. I have also found in the past if it seems the outflow current is a little strong from a filter, you can strategically place your largest piece of driftwood in the path of the current, which will somewhat naturally break it up so that the current gets softened some.