General disclaimer: don't do what I did unless you understand the
consequences. Otherwise don't blame me if everything dies.

That said, I'd like to present my findings on how I successfully rid myself
of a troublesome and persistent blue green algae (cyanobacteria) problem.
What I've done is not new and is actually outlined quite well on The Krib.
In fact that's basically where I found out about the treatment and the
techniques to follow. I consider this to be a form of chemotherapy. It's
not a cure, in that the underlying causes are treated differently, but the
symptoms are certainly eliminated. And the true treatment, which relies on
patience and discipline, should be easier to stick to and have a higher
probability of success.

In a nutshell, I applied regular pharmaceutical hydrogen peroxide solution
(3% concentration) directly to the blue green algae. Within a day it was
dead and gone. With careful dosing, I had no mortalities or even any
visible distress in any of the fish, plants or invertebrates. And the
biological filter was largely unaffected.

My tank has a net volume of 190 liters (roughly 50 US gallons). It's a
moderately to heavily planted tank, with the bulk of the plants being
echinodorous and crypt varieties. There's also an anubias nana, some
alternathera reinekii and a giant aponogeton boivinianus. Livestock include
5 juvenile angel fish, 4 otoclinus, 6 Amano shrimp, 8 rummy nose tetra and a
pair of black mollies.

Substrate is a mix of Fluorite and Profile. There is no added nutrients in
the substrate. Fertilization is PMDD. Lighting is 175W metal halide bulb.
CO2 is from regulated 20lb tank. Biofiltration is from DIY fluidized bed
reactor; mechanical filtration is from Magnum 350 canister filter.

Water: pH = 6.5, KH, GH = 2.0, NO3 = 3-5 ppm. Zero ammonia or NO2.
Phosphate around 0.05 ppm. Iron tends to be unreadable (i.e. less than
0.01).

Test kits include: ammonia, nitrite, nitrate, pH, KH, GH, iron and
phosphate. I have duplicate test kits from different companies for nitrite,
nitrate, pH, KH, and GH. The iron and phosphate kits are from SeaChem (as
is the NO2, NO3 kit).

The first and longest outbreak of BGA (blue green algae) actually drove me
out of aquaria for two years. It had completely taken over the tank -
killed all the fish, all the plants and looked absolutely awful. It took
two years of looking at an empty tank to get me back into the hobby again
(I've been doing this since 1988).

Anyway, I recently set the tank up again. Everything was washed and
scrubbed. I was going to do it right. Did the fishless cycle. Had plants
growing. The whole 9 yards. Anyway, the BGA started in a few weeks after
getting the tank going again. Not much, but enough to be disheartening. I
vacuumed it away and that sort of did it for a while. But then it came back
once I added fish (for obvious reasons). Too much nutrients (esp.
phosphate). I immediately went on the offensive by cranking up the plant
growth. However, I added too many micronutrients (trying to get the iron at
the desired 0.1 ppm range) and caused a HUGE BGA outbreak. Granted, the
plants loved it, but so did the algae. I had to get things under control.
I did several massive water changes. Getting the water back into line was
important, but the BGA wasn't going away. I needed to shock it so that the
plants would be able to regain dominance in the tank.

I though about how to kill the BGA. Some have tried antibiotics, but I
didn't want to go there. I read about barley straw extract, and didn't want
to wait several weeks (hey, it's Cinco de Mayo tomorrow and we're having a
party for 40 guests. The tank's gotta look good and I only had a week to do
it in). Plus the consensus is that straw extract only works on new BGA
growth - not existing. I'm not sure if that's completely true, but the time
factor ruled it out anyway. While browsing the Krib, I read about hydrogen
peroxide. Actually I remember the discussions about it several years ago,
but never paid too much attention (didn't have to then).

There seemed to be two approaches when using H2O2. One is the "dose the
tank" method, the other was a spot treatment method. I know that H2O2 is a
very powerful oxidizer and can do great damage to the ecosystem in the tank.
I didn't want to kill my biofilter, the plants or the fish; just the BGA. I
elected to be cautious and go the spot treatment route. From information I
found on the web, it seemed like 150 ppm H2O2 concentration is considered a
high but not lethal dose for fish. I elected to keep the concentration at
about 25 ppm, mostly since I was trying this out for the first time and I
didn't want to loose my livestock.

Day 1: Took water test measurements. All OK. Shut off filter system.
Measured out 100 mL of 3% H2O2. Used an eyedropper to squirt peroxide
directly onto BGA. Let tank sit for 5 minutes w/o circulation. Turned on
filter and watched. The peroxide reacted w/ the BGA and produced oxygen
bubbles. It was like a catalyst, all of a sudden all the plants started
pearling much more than normal. That lasted about an hour. At no time did
any of the fish or shrimp seem affected.

Day 2: noticeable decline in BGA. Most is dead (grayish), roughly 10-20%
remaining. Shut off filter, and measured out 60
mL. Squirted peroxide on remaining BGA. Fish all doing well.

Day 3: BGA virtually gone. Literally dissolved. Yippee! Water tests:
slight rise in ammonia (0.25 ppm) and nitrite (0.3 ppm). Fish all ok
(shrimp and MTS too).

Day 4: BGA gone. Water test back to normal (0 ammonia, 0 nitrites, 5 ppm
nitrate).

At no time did the concentration of H202 in the tank water exceed 25 ppm
(when you factor in the dilution of 100 mL of peroxide solution into 190 L).
There was a slight influence on the biological filter, but that recovered
quickly and completely. It's been a week, and there are only small
pea-sized balls of BGA here and there. Water quality is just where I want
it. All the fish are healthy and behaving normally. I continue to monitor
nutrient levels and attempt long term eradication of BGA via nutrient
balance, not chemotherapy.

My recommendation for using H2O2 for treating BGA: first, use a very small
amount and apply it directly to the algae (even if it's on a plant - that
seems to be perfectly fine to do). Shut off the filtration system so that
the peroxide stays "close" and concentrated on the BGA. It also allows for
the peroxide to be neutralized before it attacks the biological filter. I
preferred to treat the BGA over several days in small doses. I think this
helped preserve the ecology of the tank.

H2O2 dosing is not meant to remove the cause of BGA outbreak, but rather to
remove the living colonies and give the plants a chance to do their thing.
The cure is to get the lighting, fertilization and CO2 dosing in balance so
that the plants outcompete the BGA and keep it from being a nuisance.

Jeff in Raleigh, NC