Troubleshooting: Algae and slime

Why is there brown dust on my sand and glass?

A brown, dusty or rusty film on the sand, glass and rock, usually in the first weeks after the cycle. It wipes off easily and comes back.

Last checked 21 October 2026. How we check

What should I check first?

1. How old is the tank? In the first weeks, this is normal.
2. Check the RO/DI output with a TDS meter: it should read 0.
3. If it is still strong after a few months, test silicate.

What is most likely causing it?

Most likely first. The order is our reading of how often the sources link each cause to this symptom.

1. Silicate from new sand, glass and rock in a new tank. A tank a few weeks old, usually right after the cycle finishes (keeper reports, typical, check).
2. Silicate from source water. RO/DI reading above 0 TDS, an exhausted DI resin, or tap water used for top-off. UK tap water often carries silicate and phosphate (our reading).

What should I do?

1. Wipe the glass and siphon the sand during water changes.
2. Change DI resin when the TDS reading rises above 0.
3. Add a small clean-up crew once ammonia and nitrite read zero. Trochus and Astraea snails graze film algae and diatoms (Practical Fishkeeping).
4. Leave it to run its course: in most new tanks it fades within weeks as the silicate is used up.

What not to do

• Buying chemical removers for a normal stage.
• Adding a large pack of snails all at once: the extras starve when the diatoms go.

When should I get help?

Something dying right now? Go to the emergency playbooks.