Mechanical filter material;
https://intankaquatics.com/intank-bonded-poly-filter-floss-value-pack-3-pads/
There are many other options, above is just an example. It's just to trap small particles. It's the 1st stage in any filter system. In a sump a lot of people use filter socks or now a roller, all for the same purpose.
As for flow, IMHO, you want to have enough water movement so that a coral's tentacles will sway back and forth, and have surface ripples. You are, after all, trying to recreate the Ocean. Flow is important for the bacteria in the rocks to do their jobs properly.
Also, I was looking through your previous threads and saw about your hair algae problem. Did you get a clean up crew? A really good guide is to go to;
Reef Cleaners - Clean up Crews and Macroalgae Reef Cleaners https://www.reefcleaners.org
Use his 'Quick Crews' size for your tank as a guide. His website is an excellent source if info, and it is a very good place to buy from. While I myself have not had much luck with them dealing with algae, a good CUC is important to keep everything clean.
Do you use natural sea water? If you do, you might want to look at Paul B on here.
You might want to search 'canister filter' on here. There are people who successfully use them. Also search 'bacterial bloom'