Here's a little guide on how to use Plumbers Teflon tape to reduce filter flow. I have seen this asked over and over. I have also seen the baffle guide. I find it to be ugly and it does not work on all filter types. This is an easy alternative that does not affect the aesthetics of your tank.
What you need: Plumbers Teflon tape. Scissors.
1. You will need to remove your filter tube from the filter.
2. Remove basket from the bottom of the intake tube.
3. Wrap Teflon tape around the bottom opening of your intake tube. You should use half to 1/4 overlap at the entrance. As you wrap the tape around the tube and pull it will stretch and form a cover over the entrance of the tube. Warning pull too tight and it will cover the hole. You can cut a hole with scissors but no guarantee the tape doesn't come free during use. Best to have a natural hole form.
4. After wrapping 3x around the tube cut with scissors and press the end against the tube to bind it with the rest of the tape. It will not unravel now.
5. Put the basket back over the bottom of the tube. The Teflon should smash down and allow for it to be put on easily. Do not use too much Teflon or you will not be able to put the basket back on.
6. Reassemble the tube back on the filter. You are done.
Please see pictures.
Picture 3 shows what not to do.
Picture 4 shows what it should look like.
Please ask for this to be a sticky if you find it useful.