New algae? Problem


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Feb 25, 2014
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We have a 30 gallon freshwater tank that is been set up and running for about 10 years without issues. Recently we've noticed a series of brown spots appearing randomly on the glass on various sides of the tank. They probably average about the size of a quarter although they are not round and somewhat streaky. They are more like a film than a type of algae have ever seen. It's quite hard to scrape off but can be with some work.

The tank has artificial plants, gravel bottom and a hangup Fluval C4 filter which rated for a 70 gallon tank. Lighting is a full-spectrum LED and the tank has very little exposure to sunlight where it's located.

We tried taking a picture of this but it really doesn't show up on pictures very well.

Any ideas?


Feb 25, 2009
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If it easily came off with your fingertips it would be diatoms, but since it does not easily come off, most likely a type of black brush/beard algae. This is very hard to remove, you will need a scraper for the algae on the glass.

An imbalance of light and nutrients is the cause. Since you do not have live plants, you really do not have a problem with the algae per say. Scrape it off the glass at each water change (clean the inside of the front glass at every water change even if you do not see anything, it can start quite rapidly on the biofilm). I leave it if it appears on wood or rock; with live plants it can smother and kill the leaves, but that isn't a worry here.

The algae itself is not a problem in the absence of plants since it is naturally occurring due to the light and nutrients/organics. But it can be a clue that more maintenance is needed, such as larger water changes and substrate vacuuming at each, and keeping the filter clean. Organics accumulate in all these areas and they feed algae in the presence of light. You cannot have a healthy aquarium that has no algae, so don't aim for that; keeping it under control is the goal.


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