Fallen leaves on the bottom are not a decorative invention. Most small characins, rasboras, bettas and dwarf cichlids live in the streams of South-East Asia and the tributaries of the Amazon. There the bottom is carpeted with fallen leaves, and it is the leaves that make the water dark.

What leaves do for an aquarium

  • Tannins gently acidify the water and give it a tea-coloured tint. In soft water the effect is clear; in hard water it is almost nil, because the carbonate buffer pulls the pH straight back.
  • Shelter and shade. Subdued light and cover take the stress off crepuscular and shy fish, while fry and shrimp hide among the leaves.
  • Food for fry. A decaying leaf grows a biofilm with infusoria — the first food for fry too small even for newly hatched brine shrimp.
  • A natural look. Dark water and a dark bottom bring out a fish's colour: over a pale substrate the same individuals look washed out.

Leaves do not cure fish and do not replace water changes. The tea colour says nothing about purity — nitrate and ammonia are just as dangerous in such water, they are simply out of sight.

Which leaves are suitable

  • Catappa (Indian almond) — the standard choice: large leaves, plenty of tannins, slow to break down.
  • Oak — an affordable substitute for catappa, rich in tannins, with a tough leaf that lasts for months.
  • Beech — milder than oak, a good option when you do not need much acidification.
  • Mulberry, magnolia, guava — dense leaves that decay slowly; guava leaves are also readily grazed by shrimp.
  • Alder cones — not a leaf, but they work the same way and are easy to dose: a few per aquarium.

Which must never go in

  • Conifers — spruce, pine, thuja, juniper: resins and essential oils.
  • Poisonous plants — oleander, yew, cherry laurel, box.
  • Walnut — juglone, a compound that harms aquatic life.
  • Soft, fast-rotting leaves — poplar, birch, maple, chestnut: they fall apart within days and trigger a burst of decay.
  • Any green leaf picked from a branch. Only dry fallen litter goes into an aquarium: a living leaf still holds sap and chlorophyll, so it rots instead of releasing tannins.
  • Anything collected by roads, in town or on treated land — leaves accumulate heavy metals and chemical residues.

How to prepare them

Collect dry fallen leaves in autumn, in woodland, well away from roads and fields. Rinse them under running water, then either pour boiling water over them for 10–15 minutes or boil them for 5–10. Boiling water does two jobs at once: it strips dust and fungal spores from the leaf and makes it sink. An untreated leaf floats at the surface for a week.

Store them dry, in a paper bag. In a sealed jar an under-dried leaf goes mouldy.

How much to use

A working guide is one large catappa leaf (12–15 cm) per 15–20 litres, or a handful of oak leaves per 50 litres. Start with less: acidification depends on the carbonate hardness, and in very soft water the same portion shifts the pH noticeably further. After adding leaves it is worth checking the pH after a day and after a week — see the Water chemistry section for more on this.

Replace the leaves as they break down — usually every three to eight weeks. A whitish film and slight slime on the leaf is biofilm. It is normal and serves as food. Remove a leaf when it collapses into mush and clouds the water.

Who does not need leaves

Fish of alkaline water — Malawi and Tanganyika cichlids, livebearers, brackish-water species — should not have leaf litter: their water has to stay hard and alkaline. Nor is a tank under treatment a place for leaves: tannins bind part of the medication, and the dose stops being predictable.