Yikes, that looks like a vicious case of the same algae I've gotten in my hard water planted ARLC tank. It only affects certain plants, and it appears as a black surface algae on leaves and stems that does resemble rust. I suspect it may have something to do with the high amount of carbonates present in hard water because I've never run into it in my soft water planted tanks.
I think it is a variant of black brush algae because it seems to grow best in areas of low circulation where O2
levels may be lower than the rest of the tank and CO2
levels may fluctuate a bit. I've had good luck spot dosing it with Excel. I use a pipette and just squirt some directly onto the affected leaves. Double dosing with Excel for a week also seems to keep it in check or drive it back.
But here's the Catch-22: dosing with Excel in a heavily planted tank can lead to low (possibly dangerously low) nighttime O2
levels, which in turn, can boost the growth of more black algae. I've tested this a couple of times in my tank. I dosed Excel at regular levels in the morning. After lights out at night I tested O2
levels every hour (yes, I stayed up all night just to test my water) with a LaMotte O2
test kit. At lights-out my O2
levels were near 7.5 ppm
(equivalent to about 90% O2
saturation at 80 degrees F). Over the course of the night O2
steadily dropped, reaching a low point of 2.5 ppm
just before lights-on, well below acceptable (or safe) levels. My fish were gasping at the surface in the morning (which was what led
me to test the water in the first place) due to the low O2
Heres my theory: Plants take up CO2
and give off O2
during daytime photosynthesis, where light energy is stored as sugars in the plant tissues; at night the cycle reverses and the oxidative process releases the energy stored during photosynthesis by converting the sugars to CO2
and water, using environmental O2
in the process. In a heavily planted tank, this process can consume a great deal of the available O2
. When photosynthetic sugar production is supercharged with Excel's organic carbon source, even more O2
is consumed at night when the carbon in the glucose bonds with O2
to form CO2
. I suspect that Excel may drive the uptake of O2
during the oxidative process to the point where, in an environment containing a fixed amount of O2
like an aquarium, levels become so depleted that they can have serious negative effects on the lifeforms in the tank.
I have only a layman's understanding of the carbon cycle and my interpretation may be entirely incorrect. However, if my theory holds true, it can explain some of the problems I have had when using Excel. Heavily depleted O2
levels occurring as a result of the spiking of the oxidative process by Excel's abundance of available carbon lead to: 1] respiratory distress and possibly death due to asphyxiation in fish and invertebrates; 2] low and/or fluctuating O2
levels which contribute to conditions favorable to the formation of black algae. Observation #2 illustrates the Catch-22: using Excel to treat black algae may actually contribute to the formation of more black algae because of its negative effect on nighttime O2
levels in heavily planted tanks.
Repeated experience has shown me that there are two courses of action to avoid this problem: 1] discontinuing the use of Excel which prevents the dangerous drop in nighttime O2
levels after 48-72 hours have passed and the Excel has left the system; 2] beginning lights-out aeration of the tank to counteract the O2
depletion caused by Excel. Option 2 has worked wonders for me. When my lights go out, my airstones turn on and run until an hour before lights-on in the morning, providing the tank with a ready supply of O2
and counteracting the O2
depletion caused by Excel's effect on the carbon cycle. Since I've started this regimen I have not had any problems with gasping fish and my black rust algae woes have almost completely disappeared.
Wow, it sure took me a long time just to tell you to try running an airstone at night :p