Maracyn for GDA?


Lifetime Charter Member
Lifetime Member
Jan 24, 2005
Sacramento, CA
From "the Krib":
I just thought I'd share my experience here. I have had BGA in my tank for
quite a while, but it go really bad a few weeks ago - it was starting to
take over the tank. So I got some Maracyn and started adding it at less than
full strength - 4 tablets/day in my 72 gallon tank (dosage is 1 tab/10 gal).
By day 2 a strange red-brown haze had developed in the tank, but something
else had happened - the BGA was definately in retreat. I had also had a
black algae growing on the older leaves and stems of many plants which I did
not think was BGA, but it may have been, because by day 3 it was gone. The
red-brown haze cleared up by day 4, but the tank still has a whitish haze.
After 6 days of treatment I did a major water change and was pleased to find
that the water didn't have that sharp, icky smell anymore, but I also
discovered that the BGA was not totally gone. So I have upped the dose to 5
tabs per day and will continue to treat for another 5 days.

An interesting side effect is that the annoying green dust algae which I
have been fighting for a month seems to have gone into retreat too. Before
Maracyn, within 3-4 days of a water change the glass would begin to be
covered with this powdery green algae, and by 6 days would be quite thick
with it. Since Maracyn, the coating was almost invisible after 6 days.
don't know if this is a direct cause of the Maracyn, though - I have to
wonder if the Maracyn wiped out the biofilter and the plants are using the
NH3/NH4 directly and starving out the algae. I did check ammonia (it was 0)
using an AP test kit, but it's about a year old & I don't know how accurate
it is.
Maracyn and Algae

On APC is a thread about trying this to kill GDA, with what appears to be success. I don't understand how an antibiotic would be effective against an algae, but if it works, I won't complain. Has anyone else tried this, and with what results?

Tom Barr

Staff member
Jan 23, 2005
I do not think an antibiotic will work on things other than bacteria, and in this case, gram positives only.

Correlation or so called "cures" due to chance and when they have other species of algae as well, then the all the algae disappear is hardly a cure.

They just figured out they needed to take care of the plants.

I've never had much issue with GDA, but then again, I know how to care for the plants, thus the algae are far less of an issue for me.

Still, folks seek the silver bullet.

I think the APD poster has had algae issues for years.
Nice person.

Tom Barr


Lifetime Charter Member
Lifetime Member
Jul 27, 2007
New Hope, Pennsylvania
Can I pose a question or two. I find it hard to tell the difference between GSA and GDA. So lets suppose its GSA and not GDA. How much phosphate is in erythromycin phosphate (Maracyn), and could the dosage of medication be adding enough phosphate to help combat GSA? Since CO2 and PO4 seem to be the leading candidates?

Or am I just way off base in this thinking?


Lifetime Charter Member
Lifetime Member
Jan 24, 2005
Sacramento, CA
GDA and GSA are totally different, other than both being green. GDA is easily wiped off the glass, and if done under water, it releases a cloud of green "dust", all ready to recolonize the glass as soon as you turn your back. GSA is hard, can't be wiped off, has to be scraped off, with considerable effort, sometimes taking a sharp steel blade to get it off. GDA is a free swimming algae, in its early stages - GSA isn't.

The only way I can see an antibiotic affecting GDA is if the free swimming stage is somewhat like cyanobacteria, not quite an algae and not quite a bacteria. I am planning to try the maracyn today. I have a 10 gallon experimental tank going, with emersed grown glosso and some rotala in it, where after about 4 days of flooding the GDA showed up. It won't be expensive to treat such a small tank with maracyn, and I'm curious to see what will happen. (I realize I can get rid of the GDA by other means, but I like trying new things.)