another PMDD endorsement
I seem to have successfully achieved nutrient balance, for the first time
in my life, in a tank full of Rotala indica and Hygrophila polysperma. I've
had persistent filamentous algae (among other algaes) in the tank for about
3 years. I determined that the tank was always nitrogen-limited, so started
fertilizing with ammonium sulfate. As I was pretty skittish about adding
large amounts of ammonium to the tank and fertilized lightly, it remained
nitrogen limited, and phosphate concentration remained steady at about
.1ppm, while nitrate levels remained unmeasurable.
So I switched to KNO3, with which I could safely dose daily to yield a NO3
concentration of about 5ppm (that's about 900mg of nitrate ion), which is
all metabolized every day. So plant growth is pretty strong; I've cut back
on light to reduce required trimming, and now I only fertilize with about
3ppm of NO3 a day.
The tank parameters: temp 77-78F, 120 (now 60) watts of fluorescent
lighting over about 50G, earthworm castings under 3" silica sand as
substrate, KH 3-4, pH kept at 6.8-7.0 via yeast CO2. Micronutrients are
supplied with Dupla 24 to an iron concentration of .1ppm. Phosphate is
still measurable but is right at the bottom of my test kit's range, under
.05ppm. Mg supplementation is via epsom salts (concentration similar to
that suggested by Conlin et al), with K and NO3 supplied as noted above.
Algae growth has slowed if not stopped in the past month since I've done
this, which marks the first time I can remember where I have been able to
grow plants better than algae. I think the vascular plants are just
outcompeting the algae.
I guess this is another ringing endorsement for PMDD and keeping tanks
phosphate-limited. Eventually I will switch from Dupla24 to some other
cheaper micronutrient mixture. It seems to me that dosing NO3 separately is
best, as that lets them tune NO3 relative to the other nutrients more
easily. But convenience might become an issue.
I will be tearing down this tank in about a month for a move. We'll see
whether this tank remains as happy after the move. I'm tossing my current
gravel to get a better grain size when I move - any tips on preserving the
earthworm castings in the bottom half inch?
in Seattle, land of the long-winded post