mulm

mulm (uncountable)

1. The sludge that collects at the bottom (or walls) of an aquarium, consisting of fish excrement, decaying plant matter, and other assorted detritus
   • 2013, C. W. Emmens, Keeping and Breeding Aquarium Fishes, page 115:

     The point is that the filter not only removes mulm from the tank but also ensures that this matter is very efficiently bathed in a constant flow of water, which will dissolve out anything that can be dissolved.

mulm n (definite singular mulmet, indefinite plural mulm, definite plural mulma or mulmene)

1. darkness, gloom

• “mulm” in The Bokmål Dictionary.