# why is algae always appearing on the filter outlet



## Glaucus (Oct 11, 2009)

It seems that the edges of the filter outlet is an attraction for algae. Especially BBA. In contrast the filter inlet is always free of algae. Any idea why this is?


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## OTPT (Sep 27, 2010)

Why is dust always appearing on the fan blades? 

My guess is BBA likes to stick around strong current area with good light.
And there are BBA spores hitting the outlet all the times.
While inlet is nearly always placed in low light area.


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## Glaucus (Oct 11, 2009)

OTPT said:


> Why is dust always appearing on the fan blades?


I will answer your ironic question. Plastic spinning blades accumulates a static charge by the friction generated from moving the air. Static attracts dust.


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## neilshieh (Jun 24, 2010)

this is only a guess and from my observations, but i think it's because if lots of detritus and decaying matter is trapped in the filter media and the water exiting may be richer in nutrients which would be optimal for algae growth, plus it's near the light while the intake is in the water. but i only see this happen to filter where there is A LOT of organic debris.


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## Glaucus (Oct 11, 2009)

neilshieh, 
thanks for sharing your observation. What your saying is that above all, a dirty (nutritious) filter could be the culprit. This makes sense and is a clear distinction compared to the characteristics of the situation near the filter inlet. 

From my own observations, high light intensity is not really a key factor for BBA. I found that BBA grows just as happily at overgrowth (shadowy) places as it does at places with high(er) light intensity. So it must be down to higher nutritious values near the filter outlet.

On the other hand in my situation, BBA's worst enemy carbon is coming straight out of the filter in the highest concentrations since i use an external AM1000 reactor exiting the filter outlet. It does not seem to be affected by this, on the contrary!


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## spypet (Jul 27, 2007)

sometimes in this hobby it does not pay to think too much :-k
just rinse all your filter media, bleach dip your water outlet,
and brush out your hoses (if you have such a pipe cleaner).


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## niko (Jan 28, 2004)

It is well known that algae thrives in very strong currents. If you read German you can find information about why the flow in an aquarium should not be strong. Very reasonable information but that is not the end of the game.

Read the long threads here on APC about filtration and flow. What you should learn is not a magic number for optimal flow but rather get an understanding that a planted aquarium is a system that works as a whole. Flow is only one of the factors that let algae grow or disappear. Nutrients are another example.

A factor considered by itself and not as part of a system leads to confusion. Example: A nutrient poor tank is not immune to BBA. A nutrient rich tak is not a guaranteed place for BBA to fluorish. Yet - high nutrients definitely lead to BBA growth.

--Nikolay


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