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Playing around over the holidays with one of my tanks led to a
surprising observation which I thought I would report. The tank is a 40
gal. regular (36"x16"x16") with 110 watts of cf 6500k lighting (the tank
is shown in Karen's February AFM article on the hobby). CO2 is injected
at about 25 mg/l. Water changes average 10 gallons per week. The tank
is heavily planted with laterite in the substrate. I fertilize with
around 10 ml. of Tropica MasterGrow per week. Fish stock is 7
near-adult boesmani rainbows, 3 half-grown (3") SAEs and an unknown
number of otos (6 -10 is my guess).
The 'bows love to eat and I love to feed them. On average, they get a
½" cube of frozen live food (mostly blood worms) and a few shakes of
different flakes per day. The tank has historically had a problem with
green filamentous algae which the 'bows also nibble on interestingly
enough. I pull out a small ball this algae by combing my fingers
through the plants with each water change. I have long suspected that
110 watts of cf lighting is just too much light for this size and depth
tank. Reducing the photoperiod has reduced the production of
filamentous algae to some extent.
I have never tested this tank for nitrates because I always assumed that
this fish load and feeding pattern would supply all the nitrogen the
plants needed. The other day I tested for nitrates (Lamotte) and found
no measurable level! I added ¼ tsp. of KNO3 which should have added
about 6 p.p.m. A couple of days later the level had dropped
substantially, so I added another 1/4th tsp. of KNO3 and also some
K2SO4. Early observation seems to indicate less production of green
filamentous algae. The overall vitality and brightness of the tank
This has me thinking about supplementation of macronutrients and the
genius of the PMDD method of adding these nutrients as needed. None of
our commercial fertilizers for the water column include much in the way
of macronutrients. I have long suspected that we need to add more K to
our planted tanks, in particular. I recall dr. dave (Heubert's)
recommendation of 5 - 10 p.p.m. K in the water column. Unless fish food
is full of K, I know my tanks aren't getting much.
This leads me to another speculation. I have always been intrigued by
the phenomenon of tank "stalling." We have seen lots of speculation on
this. Amano, Walstad and others have discussed allelopathy through
which plants secrete substances to inhibit the growth of other plants.
Anyone who has seen a ring of Mayflowers has seen allelopathy in action.
However, I keep wondering if lack of macronutrients (or the balance
among macronutrients) might be the more general cause of this
phenomenon. The stalled tanks I have looked at (which have
micronutrient supplementation) have always left me with the impression
of a macronutrient problem. I can say from my own experience that I
have always been able to bring a stalled tank back to life with one or
another combination of nutrients.
Regards, Steve Dixon in San Francisco