Brown Dust in a New Tank: Living With (and Beating) Diatoms

In this guide

If your newly set up saltwater tank has developed a dusty brown coating on the glass, sand and rock, take a breath. This is one of the most predictable and least worrying stages a marine aquarium goes through.

What diatoms actually are

Despite being called brown algae, diatoms are single celled organisms with a distinctive silica based cell wall, and they thrive on the silicates that are abundant in new sand and rock as well as in ordinary tap water. In a freshly cycled tank, the beneficial bacteria and competing algae that would normally keep them in check have not yet established themselves, giving diatoms free rein for several weeks.

The good news

Brown diatom blooms are almost always self limiting. As the silicate supply that feeds them gets consumed, the population naturally declines, typically within four to six weeks of a new tank being set up. This is a sign your tank is maturing, not a sign that something has gone wrong.

Speeding things along

• Use RO or RO/DI water for all mixing and top off water, since ordinary tap water is often a significant silicate source.
• Wipe the glass and manually remove visible diatoms during water changes rather than waiting for them to clear on their own.
• Improve water flow, since diatoms tend to settle more heavily in still, low circulation areas.
• Add snails and blennies that will graze on diatoms, though only once the tank has enough stability to support them.
• Be patient. Chasing a diatom bloom with constant chemical intervention usually does more harm than the diatoms themselves.

An India specific note

Municipal tap water in many Indian cities carries meaningfully high dissolved silicate and total dissolved solids, which is one reason diatom blooms in new tanks here can run a little longer and heavier than what hobbyists elsewhere report. Investing in a decent RO or RO/DI unit early, rather than relying on tap water even for smaller top offs, is one of the most effective steps an Indian reefer can take to shorten this phase.

Article content © S Vejay Anand. All rights reserved.