I'm no algae expert but brown algae growing on the glass and on the surface of rocks etc is a diatom bloom. Similar to the below picture.
What are diatoms you ask?
Diatom is a very common type of algae which occurs most of the time towards the end of a tanks' cycling period. It is typically tan or brown in colour.
What causes it?
"Causes of diatoms
Virtually all new aquariums will go through a diatom bloom, which is nothing to worry about. Once they’ve reduced the excess nutrients they thrive on – particularly silicates – they will die back and other algae will move in unless limited by plant growth. In more mature aquariums, the cause of diatom blooms may be excessive nitrate and other nutrients, a lack of light – which diatoms don’t really require but which will stunt other competing plant growth – or insufficient oxygen." Lack of water circulation and direct/indirect sunlight is also known to contribute to the cause.
How can I do to get rid of it?
"Treatment - In a new tank, just wait until the bloom passes. In an older tank, do a clean-up and several large water changes to reduce the nutrient load. You may need to treat your tapwater if the problem is persistent. As with all algae solutions, heavy plant growth will out-compete diatoms – you’ll need to provide sufficient lighting and the correct micronutrients for plants. Some fish and snails will browse on diatoms, but only a full environmental clean-up will suppress them if they occur in mature tanks."
MOST IMPORTANTLY, TRY TO ISOLATE AND FIX THE CAUSE OF THE OUTBREAK OTHERWISE, IT WILL REOCCUR.
- If it is on the tank glass, just manually wipe it off but remember to fix the cause of the problem
- water change, more water circulation, lessen your feedings
- You may choose to get some otocinclus or baby bristlenose plecos of which love to eat the diatoms
Source: Algae control in a freshwater aquarium
- Brown Algae, Diatoms In Freshwater Tanks
- Algae control in the aquarium
- Types of Algae
- Freshwater Algae Types: An Illustrated Guide - Article at The Age of Aquariums - Tropical Fish
It's not that bad of a problem and can be easily resolved. Once your tank matures and the cycling stabilizes, the re-occurrence decreases. Just keep up with water changes, ensure you don't overfeed and have left over food (which causes an increase in phosphates), and you're all set.
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