If you mean it's too strong, then you could use a piece of aquarium sponge and place it in the filter valve where the water is being sucked in. Just leave some room at the top for water to stil flow through and this will make it less strong. I did this for my betta with a 5 gallon filter and it really works....just make sure there is enough flow that keeps the surface from becoming oily.
I'm not sure if that is accurate, but that is what happened to me because the flow is so low. ^^;
Or, you could leave it without a sponge there and just place a sponge (using rubber band) around the outflow of the filter.
Don't use a sponge that's not for aquarium use.