Huh, OK, here you go:
When I started the tank, I reserved a spot for a biopellet reactor because of all the GOOD things I'd read about them. Things like coral feeding off of the bacterial biomass that the pellet reactor produces. A "remarkable and significant drop in nitrate" as I remember one article mentioning. The reasoning about why biopellets SHOULD be good at what they do seems like perfectly sound logic....and it probably does work for many people.
BUT...it didn't work for me. Let me tell you what I used, what I observed, where I think I went wrong, and I'll let you make your own decision.
I chose a TLF reactor with the biopellet "conversion kit", and I mated it with a 250gph powerhead. I used the recommended dosage of TLF biopellets for the starting dose. And I discovered that, with the pump running wide open, I got decent tumbling in the reactor tube.
Within a week, I started to see a clear, stringy "algae" on all my rocks near the water's surface. It wasn't quite algae, and it wasn't dino either. In fact, they were bacterial mats....growing on the rocks in the DT, and in the sump. It was stringy and snot-like, and it wasn't terribly pretty. Meanwhile, the reactor and the powerhead had started to show problems. The powerhead wasn't tumbling the pellets any longer, and the bacterial mats were also growing on the screens inside the reactor, further slowing flow through the tube. I could shake the reactor, releasing clouds of bacteria into the tank, but the pellets would clump up again within a few hours or less.
After a couple weeks of this I realized another issue: Some of my SPS corals looked pale and inconstant rather than dark and vibrant. My Bird of Paradise and my Green birdsnest were the biggest offenders here; these had been deeply colored before the biopellet reactor was running, and they were faded back rather badly by this point. I was still getting growth, but color? Not so much.
By this point I said "Eff it," and I shut down the reactor. It was too much hassle for what it was worth (for me) and I didn't have the funds or desire to try some different options.
NOW.....if you try biopellets, here's what I think I did wrong. First, with all due respect to Julian Sprung and his famous TLF reactors....I don't think they're the right reactor for the job when it comes to biopellets. Too easy for them to gather and clump. Look for one that has an angled bottom, where the pellets can't all gather at once. This might also maximize the flow. Second...whatever pump the reactor says to use.....go BIGGER. People say these should "tumble;" I say they should be swirling around in the reactor as much as possible. Don't give them a chance to bond and clump. And finally, make sure you're skimming wet. This is the only way I can see to get the bacteria mats skimmed out before they become matted to your rock.
Remember, this is usually used for Nitrate reduction. Just as we do with every other nitrate tool at our disposal - Macroalgae, water changes, etc - the end product needs to be HARVESTED in order to really export the Nitrate. In this case, "export" means "skimming."