Brown Diatoms on New Reef Tanks — Normal or Not?

Brown dusty coating on sand and rock is common in the first few weeks to months of a reef tank. Diatoms feed on silicate and other nutrients present in new systems. They often fade as the tank matures, CUC grazes, and water changes export nutrients.

Manual siphoning during water changes helps without nuking the cycle. Avoid aggressive scrubbing that clouds the tank daily — patience plus stable salinity usually wins.

If diatoms persist beyond three months with high silicate tap water, review RO quality and phosphate sources. Log when diatoms appeared and cleared in ReefDiary maintenance notes.