I currently have a Reef Octopus BRH100 external Bio Pellet Reactor and I've learned the hard way that it's design sucks.
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It is a pain in the butt to put pellets in. You have to unscrew the top and pull out the whole internal structure to add pellets. The problem is when you go to stick the tube back in to reconnect the top it tends to jam Bio Pellets in to a part of the bottom where they will rot. The only way to get around this is to turn it on its side so the way is clean but that ends up making some pellets spill out. If you have water already in the tube it becomes a mess.
It also tends to clump a lot so I end up having to shake it fairly regularly to keep the flow. I have a Mag 7 pump attached so the problem isn't a weak pump. Its because the disintegrating pellets plug up the tube entrance. I am looking at replacing it with a BioChurn 90 since there is no tube to worry about and with the 90 model the Bio Pellets can't plug up the pump. Matt at Sho Tank talked to the manufacturer and there are some issues with the larger models that Coral Vue is in the process of fixing. I confirmed that with Coral Vue. Coral Vue said the problem doesn't affect the 90 units but I am waiting to see if that pans out.
You also need to remember that the recommended amount of Bio-Pellets for any tank is usually too much. Matt said the best thing to do is use 1/4th of what is recommended for your tank.