I used a trick that I found online when I bought my new betta and he hated the heavy flow of my ac 70 in my 20 long. I consists of buying a 1 liter soda bottle, cutting off the top and bottom rounded parts so you just have the remaining cylinder, slicing that so you can uncoil it, and then placing on top of the filter and letting the coil wrap under the outflow. Worked like a charm.

Here's a link to where I found it online.