I use spherical biopellets from
@Morphology. I put pellets and spherical seachem matrix carbon in the reactor at the same time and I have some frag plugs down the bottom of the reactor turned side ways (so they make little air pockets ) around the base. I have enough flow from a really cheap crappy reactor to reduce my nitrates from 50 to the approx 5 that it is now. Couldn't have done it any other way.
The things I have learnt are that you need to have the carbon to stop the pellets sticking together. It needs to be spherical or the stick type pellets get stuck in the reactor air holes. You need the T shaped frag plugs around the bottom to provide some air holes to allow the flow through. I arranged them so they were alternating the way they faced around the circle. One stem to the centre, one to the outer edge, one to the centre, one to the outer etc. always leaning like a triangle so the flat top can't block the bottom.
Then you just add pellets over time. Start with 1/4 the max rate and leave a few weeks between each addition until you're at max strength. If you overdose, you reduce the available oxygen too much and everything will die. I found you also end up with a white film all over the inside of the glass. When my reactor looked like it was sluggish and too blobby inside, I changed the carbon and gently rinsed off the beads (the pain in the arse part was carefully straining them and separating them, but it was possible). The pipes for flow were full of what can only be described as snot. I backed off with the beads, returning only the equivalent of a quarter dose for a few weeks before adding more.
This is just what I've found is an effective way of saving money and not needing to buy a recirculating reactor which I currently can't afford. It does the job.