Diatoms and not as bad as many picture. Diatoms are a brown algae that typically appear in a tank which has just completed its cycle but they can also appear in an established reef tank and looks ugly but in most cases are harmless and feed mainly off of silicates as well as dissolved organic compounds, phosphate and nitrates. Tap water can contain silicates and is a good way to get a bloom if you use it to mix salt or to replace water that evaporated from the tank. Using an RODI unit helps, although you can still get diatoms with an RODI unit if the cartridge that removes silicates becomes expired.
Diatoms are typically harmless and can be beaten once their food source is gone.
For major outbreaks you may want to consider a three day blackout. They are easily wiped from the glass with a mag float, a turkey baster or a toothbrush can cover other areas of the tank. Be prepared for them to re-establish themselves quickly after a cleaning as they are likely to be able to reappear and have re-growth rates.
To prevent their return, do regular water changes, keep the bedding clean, don’t overfeed tank, ensure your skimmer is running at an optimal level and keep filter socks and sponges clean on a regular basis.
Some cleaner crew to help control it are : Cerith snails, Nerite snails and Trochus snails and also Astraea snails are effective at removing diatoms.
I couldn’t even see my rock at peak hair algae.
