I'm presently using the Nitraguard cubes. I much prefer them to the biopellets for two reasons:
1. They are relatively cheap compared to the pellets. 40 bucks for a year's supply at the rate my tank is using them
2. I believe the bacterial action is actually taking place to some degree on the cube, as opposed to in the tank.
I have biopellets that I accidentally dumped in to the skimmer chamber of my sump over 2 years ago. They're still there. They haven't shrunk, changed shape, nothing. To me this says there's very little bacterial action taking place on the pellets themselves, I think the tumbling of the pellets in the reactor wears them down and releases microscopic quantities of whatever plastic they're made of in to the water (which would explain why it takes so long for them to start "working"). With the biocubes I'm sure there's still some amount of sloughing, but they're made from a much spongier material that actually seems to biodegrade. Within a couple of weeks they change colour and start to shrink – think a block of swiss cheese melting from the inside out. The ones that have been in there the longest look nothing like the cubes that originally went in went from a mixture of red/brown to brown/dark brown.
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