White scale in the aquarium: identification and removal

A white film on the glass, equipment, and plant leaves is a common beginner complaint. The term covers three different things. Each has its own fix.

1. Limescale (calcium)

Hard white patches along the waterline and on evaporating surfaces. It's calcium carbonate precipitated from hard water as it evaporates. No biology — pure chemistry.

Removal:

• Outside the glass — 9 % vinegar on a rag, leave for 10 minutes, wipe.

• Inside the glass (with fish temporarily moved out) — mild washing soda on a sponge.

• Prevention — wipe the waterline at every water change.

2. Diatoms (brown-white "dust")

In new aquariums, a film appears in 2–6 weeks. In light it looks light brown; in shadow it can seem whitish. These are diatoms — silica algae.

The cause: excess silicates in fresh water and a lack of "useful" competitor algae.

Fix: wait. Diatoms pass on their own in 4–8 weeks once balance is reached. Things that speed it up:

• Add otocinclus or nerite snails — they eat diatoms.

• Active plant growth — competes for light and nutrients.

• Lower silicates with activated carbon.

3. Biofilm on hardscape and equipment

Fresh driftwood and ceramics get covered with a white/gray slimy film in the first weeks. These are saprophytic fungi and bacteria feeding on the wood's leachate.

Fix: scrub it off mechanically (toothbrush, aquarium scraper). It disappears on its own in 3–6 weeks. Otocinclus, shrimp, and apple snails eat it readily.

4. Saprolegnia (fungus on fish)

A different beast — but visually similar. White "cotton" patches on a fish's flank or fins are a fungal infection requiring treatment. Details in the diseases article.

When to worry

Right after setup — never. White film on a stable aquarium does deserve attention:

• If the film grows fast — check feeding (too much food = too much organics = biofilm).

• If on otherwise healthy plants — investigate macronutrient deficiency.

• If on a fish — quarantine and treat fungus.

A white film by itself is almost always cosmetic. What's dangerous lives next door: high ammonia, fish disease, CO₂ imbalance.