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- 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” in The Bokmål Dictionary.