I am currently in the experimental process with Seachem's De*Nitrate. De*Nitrate requires a flow of 50gph or less in order for the anaerobic bacteria to colonize. That presented a problem as all my filters and powerheads push far more than that.
So I took an old Aquaclear 50 I had out of service and filled it with a filter foam on the bottom and stuffed 500ml of De*Nitrate into a media bag on top of the foam. Next I took the impeller from an unused Aquaclear 20 which is rated at 100GPH and trimmed two of the impeller vanes off opposite each other. The motors on these AC20/30/50 are all the same, it's the impellers that are different. My assumption was that if four vanes pump 100GPH then only two vanes should pump significantly less.
Bingo! I did a flow test to check volume vs. time and at full blast it's pumping at 45gph. Perfect for De*Nitrate. I even keep it dialed down a bit so it's probably only passing 25gph.
I'm keeping weekly records on my nitrate and water changes so I have a baseline to compare. It will take some time to establish the anaerobic bacteria colony and I'm only about a week in right now. I'll follow up in a month with my results.