Most people who have ever kept a betta in a small filtered tank (say 5 gallons or less) know that strong water current can be a problem. My betta is in a 5 gallon tank and I have an AquaClear filter with an adjustable flow. Even on its lowest setting the current was still too much for my betta. And his fins kepts getting stuck on the intake valve!
I went out and bought another filter designed for "delicate fish who prefer a lower current" and while my betta was much happier with the calmer waters, I don't think the filter was doing its job very well.
I did an internet search and found a cheap/practically free way of further adjusting the flow rate of my AquaClear, and modified the plan slightly to suit my betta's needs. First cut off the top of a 500mL water bottle and then slice it vertically. This will be the "shield" that you can silicone to the intake vale without affecting its suction (silicone the mouth of the bottle just above where suction begins so that the rest of the bottle covers the intake). Be sure to put a line of silicone along any of the cut edges so your fish won't cut himself!
Now take the other part of the water bottle and cut off the bottom so that you have a tube. Cut one side of this tube to make a "C" shape. This is what will be attached to the filter output to slow down the waterfall. I even used hole punch to put four holes in the plastic to disperse the waterfall. I have just put the filter back on the tank and so far my betta seems OK with the flowrate, and I'm much happier with the AquaClear back on the tank. I've attached some pictures to help explain the steps a little better.