# Why is Green Dust Algae only on Lower Half of Glass?



## happybats (Feb 1, 2014)

Hey guys, I have a mild case of GDA on my glass. Been battling it off and on for a few months. Just curiously why it is only on the lower half on my glass though! It's like a perfect line that stops about half up. Thanks


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## BuddhaBoy (Jun 12, 2014)

Light is probably too bright for it above that line.


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## shrimpletess (Jun 1, 2014)

My guess is the flow, surface agitation in the upper level. If it was the light, it wouldn't be a perfect line. 
I've noticed the same.
Dust algae, diatoms, they've got problem hanging on when flow disturbs them.


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## Raymond S. (Dec 29, 2012)

This is likely true IF it is GDA. But if it is GSA, it starts where the light starts hitting the glass towards the top of the water. I have a very distinct line about two inches below the top rim of the tank in mine. If I move the light further back, the line moves down.
Not that I prefer looking at that algae, but if I don't let it get more visible I always seem to miss parts of it when I scrape it off.
For the first two weeks, it isn't much visible. But by the third week I can plainly see it and don't miss near as much when I clean it.


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## antbug (May 28, 2010)

I love all the guesses, haha.

The line is due to water changes. The glass that is exposed to oxygen is clean, while the glass that stays under water has algae. If I were a betting man, this list is right about 50% of your water if you follow EI or what ever level you change your water at.


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## shrimpletess (Jun 1, 2014)

antbug said:


> I love all the guesses, haha.
> 
> The line is due to water changes. The glass that is exposed to oxygen is clean, while the glass that stays under water has algae. If I were a betting man, this list is right about 50% of your water if you follow EI or what ever level you change your water at.


No, the line above which the tank is clear of algae is underwater, and the clear of algae part is underwater too. The tank water does not evaporate exposing the upper glass.
The upper part of the tank looks clean as if wiped perfectly above the line.

I've also had the same happen in a smaller tank that had green dust algae. No GDA was present where the filter flow hits the opposite glass all the way down. I'd presume again  the flow pattern is just different, hence the lack of GDA appeared in a different "shape".


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## 180g (May 20, 2014)

shrimpletess said:


> No, the line above which the tank is clear of algae is underwater, and the clear of algae part is underwater too. The tank water does not evaporate exposing the upper glass.
> The upper part of the tank looks clean as if wiped perfectly above the line.
> 
> I've also had the same happen in a smaller tank that had green dust algae. No GDA was present where the filter flow hits the opposite glass all the way down. I'd presume again  the flow pattern is just different, hence the lack of GDA appeared in a different "shape".


so you don't do water changes?


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## shrimpletess (Jun 1, 2014)

180g said:


> so you don't do water changes?


 I do, 50% weekly, the tank gets exposed way below where the algae line.


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## latchdan (Sep 7, 2007)

Did it look like this?



How I got rid of mine was, reducing photo period, went from 8 hrs to 6hrs and getting some fast growing plants.


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## Hoppy (Dec 24, 2005)

What specific light are you using and is it sitting on the tank or suspended above the top? It may be that the upper part of the glass gets very little light, and algae need light to grow well.


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