mulm

(This etymology is missing or incomplete. Please add to it, or discuss it at the Etymology scriptorium.)

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.