Depending on what type of filter is is, I took a white sponge meant for filter media, cut it thin and put in over the outflow of water with a rubber band in my 5 gallon. I just did it today but it slows the water down perfectly and doesn't hurt with cleaning the water. If anything in the last 10 hours, my water is now super clear in the tank.
if you buy a sponge just ensure it does not have any type of antibacterial things in it as it will kill your cycle. Aquarium sponges from a pet store are probably the safest way to go.