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Fish/Fish Aquarium mold?


QUESTION: Hi, I have a 75 gallon fresh water aquarium.  I've had it for about 8 - 9 years.  I used to have ciclids but they eventually mostly died and I moved the last one to a 10 gallon tank about a year ago.  However prior to moving it, I had a problem with a dark redish color mold or aglae or something growing on everything in the tank.  After I moved that fish to the new tank I completely cleaned the 75 gallon tank. I removed everything including the gravel and pressure washed it and left it in the hot sun to dry for about a week.  I refilled the tank, replace the filters, everything.  The water appeared to be beautiful for a couple of months.  I replaced the fish with some community fish that I honestly don't remember what they're called.  The aren't ciclids or goldfish though.  Just little pretty blue and red flourescent looking fish and a couple of others.  There's like 10 1 - 2 inch fish in this great big aquarium.  Anyway, now I have this extremely black mold looking stuff all over everything except the fish.  It grows on the top of everything, so I'm guessing it has something to do with the light. It's very black and thick and doesn't come off hardly at all.  I don't know what to do.  I tried some phosphate strips and tested the water.  Nothing.  The sample water didn't turn yellow at all.  I'm totally at a loss at what to do to get rid of it.  The fish seem to be okay but the aquarium looks terrible and this black stuff continues to grow all over everything.  See Attachment.  Suggestions?

ANSWER: Do you have your aquarium beside a window or near a window? How many hours a day is the aquarium lights on?

Its red algae which is a problem for a lot of aquarist. There are a lot of factors of where you could be getting it.

Sometimes too much artificial aquarium light, normal light hours should be 6-8 hours a day, and off completely at night. This is healthy for the fish and the tank itself. It could be sunlight as well. If the tank is placed near a window or in direct sunlight algae will grow in hideous amounts as sunlight is literal caffiene for all kinds of algae.

Red Algae grows from spores so where are you buying your fish? If the petstore tanks's have red algae and you bought the fish there it will spread to your tank. How? The fish eats the red algae in one tank and when you bring it home it produces wastes and the algae spores in the poop grows and lives on your glass feeding off your aquarium light.

Now onto to curing your nasty red algae. You can try scraping it off with a sharp razor and see if it will come back or you could try live plants. Get a good amount of gravel substrate, pick out the healthiest ones and plant away. Live plants will out-nutrient the algae and eventually it will die and became easily scraped off with a razor.

The last resort and I am mean last, is chemical treatment, algae hates copper but you will first have to thoroughly check the fish you have in your aquarium as some fish are extremely sensitive to copper and will die. Normally scaleless fish such as loaches and eels etcetera are susceptible to it.

Best of luck!

- Ash

---------- FOLLOW-UP ----------

QUESTION: Thank you for the information.  I'll answer your questions.  First of all the light.  The aquarium is not near a window but I do have the aquarium light on for about 10 hours a day.  Apparently that is too much?  I will shorten that and see what happens.  I will first of all scrape it off of the glass and hope the algae doesn't come back, at least on the glass.  Next, I failed to mention it, but when I last set up my aquarium and purchased the new fish I also purchased 4 good sized live plants just because I thought they looked better than the plastic kind.  They died as soon as the algae started growing.  It grew on the live plants and I eventually pulled what was left of them out and tossed them.

I will try not to resort to the chemicals but if I need to, is there a particular product containing copper that you recommend?

Thank you,


Try reducing the lighting to at least 6-8 hours. Since the algae is bad it'd be good to kill them off with 6 hours of lighting. Attach the aquarium lights to a timer so you won't worry.

Also try to overfeed. I do not know how much you feed your fish but overfeeding and lack of water changes puts Phosphates in the water which is food to algae.

I would avoid chemical products bought at local petstores. I have had several boughts of red algae myself and it seems to work. This will sound dangerous but a very WEAK 5-10% amount of bleach on a rag and press/scrub the red algae will help but make sure it is dilute.

Red Algae, when it starts to grow, will grow on live plants. So please scratch what I said above. I was saying that for all algae but red algae is different.

- Ash


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Ash WolFF


My knowledge of fish ranges from freshwater and saltwater fish compatibility and care from easy livebearers to monster fish. I can help you set up a simple freshwater aquarium to a complicated reef tank. I would be pleased to assist anyone who is new to the aquarium hobby.


I have been in the aquarium hobby for eight years mostly dealing with large and small aquariums. I keep mostly monster fish (Arowana's, Large Cichlids; Oscars, and freshwater/saltwater eels specifically) but now that I am in college I am keeping a 30 gallon tropical community and a 45 gallon brackish aquarium while my father tends to the big monsters I left behind.

I'm on YouTube! My 30 gallon community posts are the only ones that are up so far. My Brackish tank is next! I'm on Yahoo Answers:

I am currently in college aiming for a job in Marine Biology.

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