Personally, I don't think there is a reason to have a DSB this day and age. We have many other options available, and have learned a lot in the hobby since the days that DSB were standard. Imo, DSB are too fussy, too easy to do improperly, and may or may not crash even when it seems they are done properly.
I'm also not a fan of these Bio-Pellets. I haven't tried them myself, but it seems the success rate is 50/50 which I don't think is good enough, especially when there are alternatives that have a better success rate. Everyone's looking for a quick fix though, and this makes the Bio-Pellets an easy sell.
Denitrification should be handled by your live rock. If you're getting a build up of nitrate you need to look for the source. The source can usually be found from over-stocking (and/or over-feeding), poor maintenance (and/or insufficient skimmer), and/or poor circulation. The source of nitrate is
always organic breakdown. The purpose of a protein skimmer is to remove organics before they have a chance to breakdown. The purpose of powerheads is to keep organics suspended in the water column so they make it to the skimmer. Settled detritus results in organic breakdown.
Instead of using a refugium and/or DSB (in tank or refugium) I would suggest the use of a chaeto chamber in the sump instead. Provide a large chamber in the sump, provide intense low Kelvin lighting, stock it with a ball of chaeto, add a powerhead to keep the ball rolling (literally), and keep the ball trimmed so it is always freely rolling, and this method will absorb an astounding amount of nitrate and phosphate. The catch is keeping the ball rolling. Most people don't do it right, but if done properly it is quite impressive.
There is a line where over-stocking becomes the problem. No matter how much circulation you provide, no matter how much chaeto you can grow, you just can't stay ahead of nitrate. You may not be over-stocking the tank, but you may be over-stocking the protein skimmer. Lots of fish means lots of waste. Lots of waste means you need an efficient skimmer to remove this waste. If you have done everything else I have mentioned and you still struggle, take a look at your skimmer, maybe you need a bigger (or better) skimmer to keep up to the bioload you are putting on it. Or less fish.
