Filter material for Fluval U4 aquarium filter.

This is to replace all the filter materials in a Fluval U4 aquarium filter and make use of as much space in the filter as possible.

This is optimzed for good biological filtration, which typically what is important.

You can choose if you want to prioritize quick start of biofiltration or long service intervals and easy cleaning, by making it more or less dense/coarse.

A fine filter starts "working" quicker, a coarse filter needs less frequent cleaning, and is easier to clean.