Nasty Black Algae - Help please!

For the past week, I've been getting some really nasty, black-looking algae growing in the crushed oyster shell I use as a substrate. It grows really quickly--I'll try picking some off and burying the rest, and the next day it's just about as bad as when it started. Any way to kill this stuff without removing all the substrate?

I'm guessing I'm committing one or more of the cardinal sins--overstocking, overfeeding, and/or too much light. The regular green algae that grew on the glass never bothered me much. I'd clean it off occasionally but figured it was actually doing some good as part of the system. It never grew terribly fast, either. This stuff looks like creeping death, however, and it is EXTREMELY prolific. Also, I'm starting to see some nitrates and nitrites again, when they had gone to zero a few weeks ago. I'm thinking part of the reason may be that this stuff is cycling so quickly that it is creating lots of dead matter for the wrong sort of bacteria to feed upon.

Any ideas?

BTW, for any who don't know me, it's a brackish system.

I've never run into that. You might try over at Wet Web Media, they have some brackish gurus there.

Cyanobacteria can be a very dark green, so much so that it can look blackish. Google "cyanobacteria and aquarium", and see if the pictures look like what you have. I find that it is often the result of excess phosphorus, combined with strong lighting. Can be very frustrating to control sometimes. A short term solution is to treat it with peroxide or antibiotics. Peroxide generally works well at a rate of 1 oz per 10 gallons, especially if squirted right onto the bacteria. It is very short-lived, and will not affect your ammonia cycle. Some plants can be sensitive, however, like hornwort (Ceratophyllum). But most are not.

For longer term control, see if you can reduce the phosphorus load by water changes or phosphorus-binding filter additives. If your municipality adds phosphorus to the water (many do), water changes can make the problem worse. When I lived in Georgia, I would up reconsititing DI water to use in the tanks, since mine were brightly lit and the tap water had a lot of phosphorus in it.

Thanks for the tips! Here's some conflicting advice, however, from an article on the bacteria/algae:

"Like a lot of bacteria, these type of algae in your substrate are beneficial to the health of your saltwater aquarium. They feast on detritus (organic wastes).

They form a dark brownish-red gelatinous mat called red slime which can result from poor water quality, high nutrients level and poor water circulation.

They can form on rocks, gravel, and plants in your saltwater aquarium and produce toxins that poison the saltwater aquarium fish. They must be physically removed with a siphon during a partial water change."

Starts out calling them beneficial, then says they're poisonous! I'm going with the poisonous verdict and trying to eradicate.

It looks like what you saw was regarding saltwater aquariums. I'm pretty sure that saltwater and freshwater aquariums have different cyanobacteria issues. "Red slime" cyanobacteria seems to be the big problem in salt, whereas bluegreen seems to be more of an issue in fresh. With you having a brackish tank... I have no idea which system it would be more like. But since you said it looked black, I might assume it is bluegreen. Maybe look at photos of each on the internet. I have no experience with the treatment of red cyanobacteria.

The red can also be very dark and look nearly black depending on lighting conditions. I find it to be nearly exactly the same as freshwater cyano in behavior. Even with extremely low levels of nutrients according to water tests the stuff seems to thrive. I'm assuming it is very effecient at removing whatever it needs from the water before the test kits tell you it is present. I've never seen it kill anything personally, unless left covering a photosynthetic organism for a long time, but it can look pretty awful.

I fought it for months in a newly setup and heavily lit marine tank with no real luck. I had a few different binding chemical medias along with lots of physical removal and it slowed down a little, but was still very unsightly and too prolific to rely on physical removal. I ended up using an anti-algae chemical, and it seemed to work well. Cleared it up for a few weeks the first time, and then it never recurred after the second application. I'm assuming a big part of the long term solution was simply the aging of the tank as it reached a more balanced state. I think the chemical I used was called chemi-clean or something cheesy. I'll have to dig through my fish closet and see if I can turn it up.

I had similar experiences with a very dark green cyano in a soil/sand substrate tank that I set up. It was partially being fed by some elodea that kept dying off, but again I think it was just a bit of a balancing act by the new tank that finally resolved it. I simply relied on physical removal and left the tank looking like poo until it finally cleared itself up. I still have a few hints of it in the most strongly lit areas, but it isn't enough to trouble me.

Good to hear it's not likely to be a catastrophe. Thanks!

Since your tank is brackish I would try raising the salt levels higher to maybe kill the stuff. the fish you have can live in full marine if you wanted.

Since your tank is brackish I would try raising the salt levels higher to maybe kill the stuff. the fish you have can live in full marine if you wanted.

Use red slime killer, its a cynobacteria algae killer and it will work.

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The stuff seems to have died back. Unfortunately, I did several things at about the same time, so I can't be sure what made the difference or, of course, whether it was just natural tank cycling. The two things I did were to cut back on the natural light by closing the blinds behind the tank and to remove some stock--my snapper that was stressing out the other fish (when he wasn't eating them).

The decision point for the snapper occurred when he tried to eat a teensie new blenny I had just added. I honestly can't believe the blenny survived after the snapper battered him around so severely, but he did.

Both actions likely helped the situation clear up. When excess nutrients are present, lowering light levels can definitely help. And that snapper surely contributed a lot to the total waste load!