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A consensus has emerged on the Conlin and Sears
PMDD formula: mix with water to make a 1/2 liter solution,
1 tablespoon chelated trace mix
1 tablespoon MgSO4+7H2O
2 tablespoons K2SO4
1 tablespoon KNO3 (may vary, depends on nitrate)
[Thanks to Steve Turner and Onis Cogburn for mix-fix]
Now how about dosing? Does anyone have any comments
on these revised sections of my previous "PMDD: Sources and
H) How to dose PMDD
1) So, why not just pile on the PMDD so you're sure you have
more than enough of each of its nutrients? Because:
- Dosing just a little bit too much PMDD can lead to a nutrient
excess that will give unwanted algae a foothold. :-(
- Too much is not the best for your fishes' well being.
- Too much of specific nutrients can be injurious to specific
- Way too much and the excess can actually inhibit a
plant's ability to properly utilize nutrients present. How's
that for contrary!
Conclusion: MORE PMDD IS NOT NECESSARILY BETTER
2) No question, many consider determining PMDD dosages
an art. The reason for this is people's requirements
for both "amount" of nutrients and "ratios" of nutrients
to one another can differ. Why? Well, here are some things
that make the amount and ratios of nutrients you need
somewhat unique to your situation:
- Nutrients already in your tap water and how often
you do partial water changes.
- The number of fish you have and nutrients generated
by them (this especially impacts N (nitrogen)).
- How much you feed your fish. Overfeeder? Minimalist?
- Fertilizers you may use in your substrate that could
leach into the water.
- The density of plants you have. More need more.
- The density of plants you have. More need more.
- The kind of plants you have. Some rely more on water
bound nutrients, some derive more from the substrate.
- "High Tech" supplements like CO2, lots of light, etc.
[Thanks to Pat Bowerman for input]
3) As a starting point, here's a three point plan for setting
an PMDD doses.
a) The amount or "size" in ml (milli-liters) of your daily
PMDD dose should be eventually governed by monitoring
Fe (iron) levels that accumulate and equilibrate in your
aquarium over a period of time (more on this later).
But, to get started, for each ten gallons, try around
1/12 ml of PMDD a day (roughly 2 drops). A 29 gallon
tank would then get 1/4 ml a day (roughly 6 drops).
If you don't do many partial water changes, dose less.
If you have a "high tech", densely planted, CO2 injected,
VHO lit tank, you might want to double the dose to
1/6 ml of PMDD per 10 gallons a day (roughly 4 drops).
b) Most of the PMDD nutrient ratios set by Conlin and Sears
will work for a variety of conditions. PMDD has been
deliberately formulated so that if aquarium Fe concentration
is proper, other nutrient concentrations should fall into place.
Really, the only common exception to this is NO3 (nitrate);
you may need to tinker with the ratio of KNO3 to other
PMDD ingredients. Adjusting this ratio will eventually be
governed by NO3 levels that accumulate and equilibrate
in your aquarium over a period of time (more on this later).
To get started, if you have around 5mg/ltr of natural
NO3 already in your aquarium, initially use the standard
1 tablespoon KNO3 per 1/2 liter PMDD mix.
If your natural NO3 is much higher, initially don't add any
KNO3 to your PMDD.
If you have next to no NO3, you might initially boost the
KNO3 to 2 or even 3 tablespoons per 1/2 liter of PMDD.
c) Lastly, as you go along, you'll need to monitor buildup
of Fe and NO3 in your aquarium and watch how plants
and unwanted algae respond. As each daily dose
adds nutrients, a portion will accumulate
until eventually you'll reach an equilibrium level.
This ramp up and stabilizing process will take
awhile (weeks) to occur. Be prepared to adjust your
dosage size and KNO3 ratio as you gather feedback.
4) The specifics on Fe
a) Consensus is to dose PMDD so that you accumulate
and maintain around 0.1mG/ltr (0.1 milli-grams/liter or
0.1 Parts Per Million) concentration of Fe (Iron) in
the aquarium. Exceeding 0.2mG/ltr of Fe seems to
With the dosage size recommended part 3, you should
see Fe levels slowly ramp up. Eventually, Fe you add
will equal what's depleted from plant use, water changes,
etc. Again, fine tune your dosage size so at that
equilibrium point, your Fe is around 0.1mg/ltr.
b) So, what if use of laterite raises your Fe levels too?
I'm not really sure though I use laterite, limit PMDD
doses to keep overall Fe levels below 0.1mG/ltr, and
still achieve ample nutrient concentrations.
c) Suspecting that to much Fe can promote algae growth,
Steve Pushak notes he only adds sufficient Fe to keep
plants growing without symptoms of Fe deficiency or
chlorosis (yellow or pale leaves), a level that may fall
under 0.1 mG/ltr Fe. He has found Salvinia to be a good
indicator plant for Fe dosing.
d) Oh, SeaChem, Red Sea, Tetra, Dupla, Hach, LaMotte
all make Fe test kits. Prices and ease of use very
widely. At the time of this writing, there has been
some controversy as to whose will accurately measure
chelated Fe; make sure yours will. Try:
"That Fish Place"; 800-733-3829 or
"Pet Warehouse"; 800-443-1160.
5) The specifics on NO3
a) Consensus is a good target for aquarium NO3 is
3 to 5mG/ltr (3 to 5 PPM).
Remember, adjust NO3 by varying the amount of
KNO3 you add to PMDD, don't adjust dose size. In other
words, the daily size (ml) of the PMDD dose is determined
by desired Fe levels. The ratio of KNO3 to other PMDD
ingredients is what you vary to bring NO3 levels to where
you want them.
[Thanks to Mark Fisher for input
b) As time passes, you might notice that as PMDD
begins to balance things out by providing other
nutrient components, your levels of naturally
occurring NO3 will fall (it's being used). You may then
have to increase the KNO3 added to your PMDD to
maintain the 3 to 5mg/ltr NO3 concentration.
Still if you do ultimately reduce or eliminate KNO3
from the mix, don't worry about having also reduced
the K. The K2SO4 seems to provide enough.
c) Many of the folks that make Fe test kits also make
ones for NO3. Check that they're accurate in the
6) I don't know what a good target for K (potassium) is.
It turns out to be really difficult to measure anyway.
Since PMDD is rich in K, and I've read you can have
a fair bit of excess K without harming aquatic life,
I wouldn't worry about it.
7) PMDD has no P (phosphate). According to work by
Sears and Conlin, having the P in N-P-K as your
limiting major fertilizer factor helps reduce algae. Of
course plants require some P (.02 mG/ltr a good target),
but you'll no doubt have more than enough supplied by
fish food and waste.
Most terrestrial plant fertilizer supplements have a
lot of P. They may also have N in the form of NH3/NH4
(ammonia/ammonium) or CO(NH2)2 (urea), both quite
toxic to fish unless dosed very very carefully. That's
why the advice to not use otherwise fine products like
8) Why daily PMDD doses rather than a weekly mega-dose?
-Some constituents of PMDD are not terribly stable
(like chelated Fe) and may loose some potency in the
aquarium over the course of a week.
-You avoid concentration "spikes" that fish might
-Even if PMDD nutrient ratios aren't perfect for your
situation, you will have at least some of whatever
your limiting factor is available each day.
9) Most drug stores sell droppers marked in mL for
dispensing medicine. The 2.5 to 5mL sizes work
great for metering out PMDD. Oh, when dosing,
I'd first shake the bottle in case of settling, then
try to stir the PMDD slowly into the aquarium so
fish don't get hit with a "chemical cloud".
Also, if you have a small aquarium and find
fraction mL doses tough to dispense, you can dilute
the PMDD and adjust dosages accordingly. For example,
use twice as much water to mix up the PMDD, double
your dosage size.
I) Some of my dosing experience
1) Did I follow my own advice and go slow . . . no. <g>
I initially dosed with 1.7 ml PMDD a day in my
45 gallon tank (40 gallon water column). I changed
25% of my tank's water each week. If you assume
no Fe depletion (a poor assumption but good for
establishing an upper bound) my Fe concentration
would have eventually leveled off at 0.4mG/ltr. Too
high! So, once Fe reached 0.1 mG/ltr (took
about 2 weeks at the 1.7 ml dose), I reduced my
PMDD dose to a little over 1/2 ml a day.
My plant growth really improved. :-)
2) Now you've read this and want to "jump start" things
too. True, I did find for the initial first few weeks
I could safely double or triple dosage recommendations
made in part 2 of "How to Dose PMDD". The problem is
you've got to keep very close track of Fe and NO3, and
then remember to roll back your dosages once you
reach your target levels. If you don't, at best you run the risk of
becoming an algae farmer, at worse your plants and fish
Tim Mullins - Pittsburgh