When incorporating biopellets into your filtration scheme, make sure that your aquarium has adequate oxygenation. During biopellet use, bacteria populations in the aquarium skyrocket. This means there are more organisms consuming oxygen, and without the proper aeration, the dissolved oxygen level in the aquarium can drop to dangerous levels. Fortunately, there are numerous simple fixes for this issue. For starters, introduce the biopellets slowly, allowing your system to adapt to the increased bacteria. Also, you can inject oxygen into the water with the help of a protein skimmer. If these two methods don’t solve the problem, you could also try increasing the surface agitation of the water by pointing powerheads upward slightly to help promote gas exchange.