cyanobacteria.... nasty


Guru Class Expert
Apr 29, 2008
I have a nasty outbreak of cyanobacteria -
Its not a large outbreak, but the cyano is there on my moss and some of the HC.
I tried everything from keeping CO2 upwards of 40ppm for a while, to lowering the lights, blackout and erythromycin.
Cyano did die somewhat in the 3 day blackout, but not completely, and is back, which cant be said for some of the plants which didnt fair well with the blackout, maybe because i have very high flow in some parts of the tank and the plants didnt have strength to withstand flow without light.
This tank was growing great plants, under hight light and without any algae, even when i had a lot of NH4 in the water.
temp in the tank is also low, and i have tried several large water changes.
Dosing is more than generous.

Like I said before, there isnt very much of it, mostly on the moss.
Dosing more erythromycin right now, I dont really know what could be the cause of this, and frankly dont know what to do.
It was a perfect tank before this, not one spot of algae anywhere, all the plants were growing great too, now i cant get Cyano out of my tank and out of my mind!


Lifetime Charter Member
Lifetime Member
Jan 24, 2005
Sacramento, CA
Blue green "algae" is a sign of high light without enough nitrate. Since nitrate at the concentrations we will run into from fertilizing doesn't harm anything, why not just increase your nitrate dosage by 50-100%?

Why did you have a lot of NH4 in the water?


Lifetime Charter Member
Lifetime Member
Nov 21, 2007
Los Angeles, CA
I've heard that erythromycin can harm nitrifying bacteria, so be careful there.

Possible causes of cyanobacteria:

1) Lack of nitrates, as Hoppy (Vaughn) already mentioned. Since your tank was doing very well, I'm guessing your plant mass increased and you were not dosing enough. If you were dosing the full EI amount for your tank, that should not have been an issue. Did you reduce KNO3 for any reason?

2) Decaying matter in your tank. You may have neglected pruning and removing old decaying leaves. Or, perhaps your filters got dirty.

Sounds like you have cleaned up your tank at this point. So, hopefully increasing your KNO3 will solve the problem.


Guru Class Expert
Apr 29, 2008
I dunno... I didnt have any decaying matter in tank.
I had NH4 because tank was cycling.
From what tom had posted, cyanobacteria is rarely a sign of lack of nitrates, unless there is none at all.
I am doing heavier dosing than most, i am not even going to bother measuring NO3, i know its up there.

btw, it seems today that cyano moved to the bottom. for some reason, its only on moss and HC - those are the 2 things that pearl the most in the tank, must be related, cyano can fix N from air, but pearling plants produce O2, so dont see how this is related.

This stuff is NASTY, i dont believe its caused by any kind of an imballance, and its NOT responding to medication, which is puzzling.


Guru Class Expert
Apr 29, 2008
yeah, i did read that one, it seems like what I have - NOT ON PURPOSE!
btw, i doubt this can be induced on purpose...... unless you have the strain or you add very little amounts of antibiotics so an immune strain can develope.

I have neverreally had trouble with any kind of algae before, this is a first, and its NASTY!

Tom Barr

Staff member
Jan 23, 2005
Low NO3 is part of it, and often the main issue in newer tanks and tanks that are really clean and prepped well.

For other tanks, often clogged filters, a lot of organic matter, dirty filters etc, seems to be more the issue. Also, if you have a lull in plant growth in general, something it will infest some plants, a bit, maybe at the growing tips.

Excel might help a little, not much, but the overall effect of a large water change+ cleaning filters, + dosing more KNO3, + tweaking CO2 a bit more, etc, jujst run down all the basic items that you need to do and check them off and make sure they are in good shape.

EM will work also. I do not think there's any species that we ever have that resistant to EM and that's really a more3 general issue, not really specific risk for us, or in general since so few folks ever have BGA to begin with.

It's when they use it in the food supply, or when folks do not take the entire doctor's prescription and the weakened TB strain is now resistant to the treatment, but you did not quite kill it all, leaving a strain that can handle more of the antibiotic or is entirely immune, non prescription (like us/pets in general), we are dooing everything wrong when it comes to management of antibiotic drugs and AIDS treatments we can if you made a list of what not to do.

Let a few few folk die today, so we can save everyone from a disease later, hard decision for the AIDS treatment.

The Antibiotic issue is far more widespread and we are using it for things that are not related to human health and infectious diseases. If they raise the chickens and cattle better, they do not need antibiotics. We have other treatments for BGA. Fish folks might get irate at the prospect of treating via the Vet MD, but it's worth while if you are the mother of a kid who has some resistant TB strain......

It's not a matter of "if they become resistant", it's really a matter of when, we can get far more out of the drugs we have if they are well managed, same for herbicides, insecticides, fungicides, antibiotics, Rodenticides etc is in every major life group pest. They all can and do gain resistance.

We select for it actually by simply running millions, billions of chances for a mutation to occur that allows the critter to evolve and avoid the toxic selective effect of the drug, herbicide etc. We essentially speed up evolution, this is far more troublesome than any GMO issue and this has actually been well proven for many years in virtually every single chemical treatment method for any and every group of pest.

Back to BGA,

Clean and clean and dose and dose, CO2 etc.
Blackout will kill what is there, it does nothing for prevention or reoccurrence.
You need to clean and add KNO3/CO2 etc.

Plenty of tanks do fine for a time, sometimes, weeks, days, sometime months, but then they break down for whatever reason, we did something wrong, find it and fix it and then get it back to where it was. Same old thing over and over.
We know where to look and what is the more likely areas.

BGA has been a nicer alga to play with since it's easy to kill with many different methods and fast to induce again. Does not bother plants too much, not like BBA and various green algae.

Green water is one of the best to work with.

"Know thy enemy."

I'm sure most have heard that, yet few know algae very well.
They do not know how to test for it or induce it in planted systems.
So little has been learned in the aquarium hobby about it.
You have to want to know about it to do that.
Near as I can tell, no one really wants to.


So not much is going to get done.learned or advanced unless I do it or someone else has the same interest as me. I've not met one person really other than Steve and he's been out of the hobby for almost 9 years.

Tom Barr


Guru Class Expert
Apr 29, 2008
Tom, the tank is rather new... so I filters cannot be clogged with anything, I checked, the one cartridge that is easier to pull out and all it has is some ADA dust.
There were a couple of plants that had a couple of rotting leaves that I removed after the blackout. BTW, blackout didnt kill all BGA, it did make it somewhat better.

I dunno, should i just do water changes every couple of days and dose a bit more than usual, say x2 amount?

I have never had any algae problems, and always thought that if plants are doing well, i wont have any algae, however, this is not the case here....
it is also strange how the bacteria always traps bubbles inside it.