Purple Algae in 20G Freshwater Tank

OuiBonjour

Hello, I have purple algae in my tank. Mostly on my rock, but also on the glass and on some plants.

When I google Purple Algae, people usually refer to Black Beard, Cyanobacteria and Coralline, but when I compare them to mine I feel like none of them look that much similar.

It is not filamentous or hairy, and it does not look slimey neither (haven't touched it with my fingers, but based on look it doesnt seem slimey like some photos of cyano).

When I brush it up, it almost seems powdery and it clouds the water. A good brushing usually removes 80-90% of it, but a pale purple tint remains at some places and the some darker spots remains on the rock. It's usually back to the point of the video in 3 days.

Here is a video of it. In the video it is vivid-purple, but in real life it's a little less vivid. But still very purple, way closer to pulple than to black or brown.

Any idea what is it ? And is it toxic ? And if so what should be the course to follow ?


Tank has been cycled for maybe a year, A+NI are 0 and 0. I don't have liquid for Nitrates anymore, but in the last two months it was between 2 and 15 when I was changing every 7-9 days. For the past month and a half, I'd say the water changes were at 13-14 days, 50%. I know it should be weekly, but you know how it is sometimes

Thanks !
 

Evergreen2

If I didn't know better, I would suggest Diatoms... Following.
 
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OuiBonjour

If I didn't know better, I would suggest Diatoms... Following.

I had some brown diatom during my cycle last year, and I agree that it's the one that looks the most like it. (besides the color). Can Diatoms be that purple ? And in a cycled tank ?
 
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Evergreen2

Purple, I'm not sure. I know depending on the silica level in your water, that you can be continuously be fighting the Diatoms despite your tank being cycled. After all, Diatoms eat the silica. I'm starting to suspect some silica in the water because one of my tanks is over 6 months old and it hasn't died down.
 
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OuiBonjour

Okay, so it's even more possible that it's Diatoms then! Now we need to know if it can be purple lol
 
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StarGirl

It looks more to me like Cyano or BBA than diatoms. Diatoms are rusty brown color. I just looked a a bunch of pics and none were purple.
 
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RoadKill

Wonder if the lightning is giving it the purple-ish hue?
 
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Cherryshrimp420

Just something that grows on certain non-inert rocks. I've had it before too, never bothered with it though
 
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