Matrix is the best biocarrier, IMO. I use it on my 5 gallon betta tank. Large surface area due to the porous composition (many microscope images on google) and you can pack a lot of it into a small space. The pieces are about the size of a small grape, and after a little rinsing to get rid of the dust, you pop them in the filter and forget about them until service. I have 250ml of Matrix on the back of my Fluval Spec V in a nylon bag under the sponge that came with the tank. I have been cycling without fish for several weeks and have enough bacteria in my tank to easily process 3-4 ppm ammonia in under 24 hours. The matrix works, and as a biocarrier nothing is simpler or better than this. For all Spec V and Spec III owners, I use this with a standard sponge (cut off a bit to shorten), standard ceramic media. , and a 100ml bag of Purigen, and it keeps my water filtered and crystal clear.
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