The biggest problem with using them is that they are NOT used properly. Most reefers are tumbling them too aggressively.
Here is an old link to Grumpy Old Reefer, who explains things well:
http://grumpyreefer.net/2010/02/08/n...-story-so-far/
I myself now use a water tank water bottle modified with a 700GPH pump that slightly tumbles them in the middle, while the surrounding pellets are slightly suspended with good water flow through them without tumbling. Every few days I do tumble all the pellets with my hand, and in doing this I can tell you that the mulm production is there and wow is it ever, even better than before when I ran a reactor (Vertex & Deltec). So this DIY reactor is the best.
With the research I have done, most that are very successful, DO NOT use the common reactors like all of us are doing. They do what I now have.... A DIY Project, A Bucket with a pump and so forth.
The KEY to all pellets is having substantial surface area for them and not going crazy on the tumbling. I myself now run about 5L of pellets in my DIY.