A BGA Observation


Lifetime Charter Member
Lifetime Member
Jan 24, 2005
I have a 20 long that I am using as a grow-out for Endler's. It contains about 25 vals in a 6" by 6" container, a little hornwort, and about 12 Endler's.

The water is moderately hard. The tank gets no CO2 or other nutrients.

The tank gets about 2 WPG of T8 light, plus about 2 hours of morning sunlight that strikes the right half of the tank.

The half that gets tthe sunlight has a luxuriant growth of BGA. I remove it periodically but it comes back. The other side has no BGA at all.

From this I conclude that excess light is the main cause of BGA in this particular environment, and maybe in others.



Plant Guru Team
Lifetime Member
Sep 23, 2007
South Florida
Hi Bill,

Could you look at it as follows:

The higher light causes a higher c02 demand. This was not met and thus BGA develops.

At the artificial light levels you are able to supply the c02 demand to keep it at bay. The sunlight caused a tip in the scales and BGA :)

So, is it too much light or too little c02? Or both? I vote for both.....


Lifetime Charter Member
Lifetime Member
Jan 24, 2005
Hi, Gerry,

What you say is true in general, but in this case the BGA is limited to the half of that tank that gets the sunlight. CO2 and nutrients are the same throughout the tank. The CO2 in the non BGA side of the tank is the same as that in the side that has the BGA. The only variable is the sunlight.



Prolific Poster
Oct 31, 2007
there will be a imaginary solution: install a bulkhead into the tank, separating higt ilumination vs low, having same nutrients levels, plants, but co2 with differents levels...
im with Gerry. are both. no balance between co2 and light.

a factible solution: install a kind of net fabric, or something on high iluminated half, to absorbe some light before hits the water.


Tom Barr

Staff member
Jan 23, 2005
I've noted this going back in the 1990's, Steve and I kept after this one.
I agree with Bill, not sure why the BGA grows where the sunlight hits more than any other spot.

The solution is easy, attach some material to block the light there etc.
EM tablets etc.

But........does not address the root.

I've seen it grow well along the edges between the gravel and the glas sin many tanks, but goes not farther.
ADA As does this after its fairly old(12 months+), most all gravels do.

Nothing disturbs it there, and it gets light.
Ideal palce for algae.

Bill's tank is different though.

Not sure why, when I uproot, do not clean filters and do not dose, I tend to get it.
Since I dose KNO3 slightly in my non CO2/no excel tank, I also do not get it ..........

Tom Barr


Lifetime Charter Member
Lifetime Member
Jun 21, 2009
Surprise, AZ
20 Longs Cause Algae

Hi Bill,

I have not found a problem with sunshine and BGA or algae in general. :rolleyes:

I may be a bit more tolerant of algae than many around here.

I have also noted more of a tendency to get algae in corners and below the substrate line on the glass.

The 10-gallon tank below is typical of my small outdoor tanks; this one gets the most sun, this time of year that is about three hours. It has a small circulation pump that powers the “breath reactor,” no filter, heavy EI dosing. It has an two inch enriched worm poop and kitty litter substrate.

The 20 long is outdoors in a sheltered area and this time of year gets about two hours of direct sun on one end. Inert substrate, low EI dosing ambient light, sits between two well lit 55-gallon tanks, gets "overhang" of shop lights.

Given your situation and mine, I believe we must conclude 20 longs cause algae. :cool: