How can I slow down filter suction in an aquarium?

You can get knee-length pantyhose and put it over the intake and outflow of the filter. This prevents the fins from being sucked into the filter and also dampens the flow so it is better for the fish to swim with. I had a Tetra Whisper 10i (a very strong filter) and had the same problem. After this was done, the Betta was able to swim around uninterrupted. You might have to supplement the filter with a few more water changes, as anything not too fine might not be picked up by it. Just replace the sock every few weeks and you should be good.

Also, you could go with a sponge filter or a box/corner filter. They are powered by air and are much more gentle for a fish with longer fins like Bettas. I have 2 box filters running with 2 of mine and they haven't been sucked in or pushed around, and many swear by sponge filters.

You can't really slow it down, your best bet is to by a new one that is slower, ask your local pet store and they will recommend a filter for you.

Good luck!:-)

Get an AquaClear filter sponge. Poke a hole in it, slide it over the intake, this is an old trick for fry that works well with heavily finned species.

For my two betta tanks I took some plastic canvas (only a few dollars at walmart and made a cylinder that was wider than the filter tube and the height of your aquarium.Use hot glue or another aquarium safe glue to create the cylinder. I push it into the gravel and have it go up as high on the filter as it can fit. If the filter current is too strong you could also cut a rectangle that you you to your filter and Pr the other end go into the water. (This also works very well for decorations, such as tubes to swim through.)