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Either black algae or mold

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The top of my decor and front and back aquarium glass had what was either black algae or black mold on it. My water levels are on point, the best it’s ever been and my temp is right where it should be. It’s a 50 gallon tank. I do 15 to 20% water changes every week cause my zombie fish is a dirty gold fish. Today I took all the decor out cleaned it, did a 50% water change. Why would this be growing in my tank that bad?  It was black.  The tank is by a couple windows but no direct sunlight to the tank and at night I do turn the light on for a couple hours. The light is a fluorescent bulb, could this cause it and if so what is the best light to use or does it not need light?  Thank you for any help!


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Was the black substance black and HAIRY...like little tendrils of hair? Or was it flat to the surface of the glass/decor?

If it was hairy, that could be Black Beard Algae, or possibly Staghorn Algae. Otherwise, it was just regular algae. I don't see how mold could be growing in your tank. 

LED lights do seem to be better for tanks. But I'm not sure the science-y reasons for all that.

Maybe @Streetwise could "shine a little light" on this question for you. 🙂  See what I did there? LoL! He's got some really great info on lighting that might give you some answers on your fluorescent lights.


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Cheers @Alesha!

I have tried to share and help with Fluval light programming. My generic advice is that you want to think about the time schedule, the power schedule, the height above the tank, the depth of the water, and the density of plants and hardscape.

Most of my algae problems have been in the very top layer of tanks with lid-level lights. I run all of my Fluval Nanos in the tall configuration so that the light is as far from the surface as possible.

I am considering creative ideas to raise my long lights.


Edited by Streetwise
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  • 5 weeks later...

When I was keeping marine aquariums, I got it a few times, and I would remove what I could, and then stir the sand to bury the rest. I don't know if what I did with the sand was recommended or not, as my only source of information was my local fish store.

If it is just on that decoration, you could simply remove it, and scrub it.

We have to deal with outbreaks on on Lake Champlain. According to the Vermont Cyanobacteria Tracker:

"Cyanobacteria grow well in water that has high amounts of nutrients like phosphorous and nitrogen. Cyanobacteria can multiply quickly to form surface scums and dense populations known as blooms, especially during the warm days of late summer and early fall."

This makes me think that you might have excess nutrients, and that heat may play a part.

I'm sure that there are other forum members with more knowledge on this topic and they should be able to add more about treatment and biochemistry.

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I did have the heat up cause my daughter bought a new fish and just dumped him in the tank and did not quarantine it, so I had an out break of ich and i forgot about the heat being up and I just turned it back down a couple weeks ago. Maybe that will help.  Excess nutrients?  What type of nutrients are you referring to? I really appreciate your help and knowledge!  This is my first and only fish tank ever, so don’t really know alot about them. 

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