JM thanks for the interesting topic to discuss.

I'm trying a pellet right now to see if I get the same results. I'm going to run it the full 15 days like in the experiment. I think many carbon should be fine if you use them properly. These are the steps I normally take and not seen HLLE.

1. I always rinse it
2. Carbon removes more when water flows through it. Not around like in a fluidized filter bed. I place it in a plain ole filter sock in the sump where water can go through it.
3 Carbon removes the most out of the water in the first 24 hours of use. I normally pull it out after 4-7 days.