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What it is
Fish produce waste, mucus and dead skin; food gets missed and plant leaves die back. All of it sinks and is slowly broken down by bacteria and tiny animals into fine brown particles. That detritus collects wherever the water is slow: under decor, in the corners, deep in the gravel and in the filter.
When it helps
In a planted tank, a thin layer of mulm in the substrate feeds plant roots, and it is home to the small life that fry and shrimp graze. A tank with no mulm at all is often one that is cleaned harder than it needs.
When it is too much
Thick heaps hold waste that keeps breaking down into nitrate and phosphate, which feeds algae. Disturbing them clouds the water, and very deep, compacted pockets can go without oxygen and smell of rotten eggs. Rising nitrate between water changes and a brown cloud whenever you move a plant are the signs. Nitrate · Bad smells
Keeping it in check
- At each water change, vacuum a third to half of the open gravel, a different part each time.
- Over planted areas, hover the siphon just above the surface instead of pushing it in.
- Rinse filter sponges in the tank water you siphoned out, never under the tap, so the filter bacteria survive.
- Aim the filter outflow so mulm drifts to one spot you can siphon easily.
- Feed only what the fish eat in a couple of minutes.