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Practical PMDD Information (Sources and Doses)

Date: Mon, 21 Apr 1997 21:38:29 -0400 (EDT)
From: Tim Mullins <>

Here's my updated post on where to buy PMDD (poor man's dosing drops) and how to use it. Many thanks to those who have made comments and helped me along.

Conlin and Sears' latest PMDD formula

mix with water to make a 1/2 liter solution,

[Thanks to Conlin and Sears for their great work. Thanks to Steve Turner and Onis Cogburn for mix-fix]

Sources for a chelated trace mix

  1. Plantex's "CSM"
    314 Orenda Ave, Brampton, Ontario
    L6T1G1 Canada
    [Thanks to Kevin Conlin]
    $30 US for 5Lbs
    For a breakdown, see How to Roll Your Own with Plantex below
  2. Miller Chemical & Fertilizer's "Microplex"
    Hanover, PA
    Distributor: NitroPhos 1-713-530-5558
    [Thanks to Doug Skokna]
    I've never tried Microplex, but it has;
    Mg-5.4%Bo-0.5% Co-0.05%
    Cu-1.5%Fe-4.0% Mn-4.0%
  3. Some hydroponics and local garden shops carry trace mixes. But, check their ingredients list to make sure they do not have N, P or K (i.e. just trace mix, not something like Miracle Grow), they're chelated, and that their trace percents are similar to CSM.

    Some have concerns with Cu (copper) being in a PMDD trace mix (Cu can be quite toxic to plants and fish in high concentrations). It is in Microplex and to a lesser extent in CSM. I haven't had any problems with CSM though I do change water each week. Many have had success with Microplex too. If you use some other trace formulation, you might want to be sure its Cu percent isn't too high.

Sources for MgSO4 (Magnesium Sulphate) and H3BO3 (Boric Acid)

  1. MgSO4 is available from drug stores as Epsom Salts. $2 buys plenty. Note it's hydrated (the form PMDD calls for) so it's labeled MgSO4 + 7H2O. [Thanks to Karen Randall]
  2. H3BO3 is Boric Acid, a Boron source used to supplement CSM (skip if your trace mix has boron). Also available from drug stores, a couple buck buys around 250mG, a lot. [Thanks to Paul Sears]

Sources for KNO3 (Potassium Nitrate) and K2SO4 (Potassium Sulphate)

  1. ECO Enterprises 1-800-426-6937
    1240 NE 175th Street, Seattle WA 98155
    KNO3 $5.98/ 1 lbs, 12.45/ 5 lbs Agro grade
    K2SO4 $5.98/ 1 lbs, 7.45/ 5 lb Agro grade
    chelated trace mix $9.95/16 oz.
    shipping across the US is $9. [Thanks to Doug Skokna and Noel Llopis]
  2. PCI Scientific Supply 1-800-432-4136
    KNO3 $16.75/500mG Lab grade
    K2SO4 $15.50/500mG Lab grade
    [Thanks to Tom Polk]
  3. Local Hydroponics and Garden Supply shops.
    Note: Greenlight's "Stump Away" is just KNO3.

Source for all PMDD ingredients, pre-mixed per Conlin and Sears formulation

Home Grown Hydroponics
On the Web
[Thanks to Suwat Phruksawan & Louis Lin]

How to roll your own PMDD with Plantex CSM

As noted by Tom Polk, Plantex's new CSM has no B (boron), so it must be added. It has no Co (cobalt) either, but I've not found the need to add any.

To add B to CSM to make CSM+B, mix 10.5 teaspoons of CSM to 1 teaspoon of H3BO3 (CSM to H3BO3 is 14 to 1 by weight). (CSM+B has a ratio of 5.5Fe(iron) to 1 B)

Here's the breakdown:

Regular CSMCSM+B Mix
Fe 7.0% 6.53%
Mn 2.0% 1.87%
Mg 1.5% 1.40%
Zn 0.4% 0.37%
Cu 0.1% 0.09%
Mo 0.05% 0.05%
B 0.0% 1.18%
Co 0.0% 0.00%

Now, using the Conlin and Sears PMDD formula, mix with water to make a 1/2 liter solution:

Some mixing notes: And yes, with the dry chemical quantities you no doubt have acquired, you could make enough PMDD to fertilize Lake Erie :).

Why to consider rolling your own PMDD

True, there are many commercial aquatic plant fertilizer manufacturers like Seachem (Flourish), Dupla (DuplaPlant 24), Kent Marine (Freshwater Plant), Aquarium Products (Flora Boost), Coralife (Iron and Micronutrient), Red Sea (AquaFlora), Tetra (FloraPride) Hawaiian Marine (Ferrovit), Jungle (Plant Saver), Wilken (FloraSan), Wiegandt (FerroPlant), Kordon (Plant Nutrient), Aqualine Buschke (Ferreal and Floreal) Tropica (MasterGrow), Aquarium Pharmaceuticals (Aquatic Plant Stimulant).

So why bother with PMDD when you can buy these?

How to store PMDD

I'd be sure to keep PMDD chemicals away from the kids ! Some are nasty.

Unmixed PMDD constituents are stable enough. The chelated trace mix should be kept dark, dry and not unduly hot.

Some have noticed mixed PMDD will degrade over time and even develop mold. Things that will help:

How to dose PMDD

So, why not just pile on the PMDD so you're sure you have more than enough of each of its nutrients? Because:


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:

As a starting point, here's a three point plan for setting PMDD doses.

  1. The amount or "size" in ml (milli-liters) of your daily PMDD dose should eventually be 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).

  2. 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.

  3. Lastly, as you go along, you'll need to monitor buildup of Fe and NO3 in your aquarium. Then, watch how plants and unwanted algae respond. As each daily dose adds nutrients, a portion of some 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.

The specifics on Fe

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 be counterproductive.

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, try to fine tune your dosage size so at that equilibrium point, your Fe is around 0.1mg/ltr.

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 seem to achieve ample nutrient concentrations.

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.

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.

The specifics on NO3

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 target 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]

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 aquarium 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.

Many of the folks that make Fe test kits also make ones for NO3. Check that they're accurate in the 5mg/ltr range.


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.


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 Miracle Grow.

Why daily PMDD doses rather than a weekly mega-dose?


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 fractional 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.

An aside

Here's another ingredient no plant tank should be without: the decreasingly rare and but still wonderous SAE (siamese algae eater fish). Believe me, these fish are a joy to watch and will earn their keep eating algae. In my opinion, you simply must have these fish!!!

Some of my dosing experience

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. :-)

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 will suffer.

PMDD's breakdown by element (standard 1 tbsp KNO3):

G/ltr % total Ratio to Fe
N 4.24 6.41% 3.06
K 41.40 62.53% 29.84
S 15.56 23.50% 11.21
Mg 2.86 4.32% 2.06
Fe 1.39 2.10% 1.00
Mn 0.40 0.60% 0.29
Zn 0.08 0.12% 0.06
Cu 0.02 0.03% 0.01
Mo 0.01 0.01% 0.01
B 0.25 0.38% 0.18
Co 0.00 0.00% 0.00
And two important quantities
NO3 18.78 -- 13.54
SO4 46.67 -- 33.63

Nerd stuff on calculating aquarium PMDD constituent concentrations

Figuring the change in aquarium concentration of Fe (or any element) after a single PMDD dose

       Increase in Concentration in PPM =
 (PMDD concentration of element)*(Dose)/(water column)
       in mG/ltr or PPM          in ltr     in ltr

my case: 1.7 ml PMDD, 45 gallon tank (40 gallon 160 liter water column), and Fe is of interest.

     [(1.39G/ltr)*(1000mG/G)]*[(1.7ml)*(ltr/1000ml)]/(160ltr) =

         _0.0144 mG/ltr Fe_  increase in aquarium from dose.

Of course, to figure it out for multiple daily doses (no water changes), multiply by the number of doses.

Figuring the cumulative change in aquarium concentration of Fe (or any element) in PPM after long term daily PMDD use with periodic water changes

The tricky part about this is knowing depletion rates. I can figure out how water changes will deplete an element, but I don't know how much plants will. Well, since my intent is to avoid overdosing Fe, I'll look at the worst case: one where no Fe is used by the plants (sure, a rotten assumption since the whole point is to provide Fe, but it does yield an upper bound or maximum possible concentration).

Turns out concentration buildup given periodic water changes is one of those Geometric Series things.

Say "D" is the total concentration increase after multiple PMDD doses preceding a water change (figured in mG/ltr per part 1). Say "R" is the portion of pre-change concentration that remains after the water change (ie a 25% water change leaves 75% of concentration).

Well, after 6 weeks, you'd have a cumulative concentration of:

         (((((D*R)+D)*R+D)*R+D)*R+D)*R+D      or
              D*R^5+D*R^4+D*R^3+D*R^2+D*R+D   or
              SUM n=0 to 5 of (R^n+D)

Now, over many many weeks you'd have a cumulative concentration of;

              SUM n=0 to infinity of (R^n+D)  or
              D/(1+R)     Simple result, aye?

So, using this secret formula D/(1+R) and putting it in terms of daily doses, days till water change, and % water change (note: % water change = (1+R)):

      Long Term Cumulative Increase in Concentration =
 (daily increase from dose)*(doses till change)/(% water change)
     in mG/ltr or PPM               in days       in decimal
                                       only valid if water volume
                                       change  >> dose volume

Carrying over the example from part 1): 1.66 ml PMDD dose daily, but with 25% weekly water changes;

            (0.0144 mG/ltr Fe)*(7)/(0.25) =

       _0.4mG/ltr Fe_ final aquarium concentration

Uh-oh, 4 times too high! That's why I ended up leveling off at a lower PMDD dose, about 1/2 ml a day, for my 45 gallon tank.

Again, this all assumes no depletion, a lousy assumption, so I'd measure Fe as you go. As I said, this calculation is only good for an "upper bound".


So, just add PMDD to your water column and your plants will grow like weeds? Well . . . maybe. There are other factors to investigate. Some feel substrate fertilization is key to success with certain plant species. Others swear by CO2 injection, or VHO lighting, or substrate heating, or laterite. If you made it this far, you're probably one of those folks that just can't learn enough about this stuff. :-) Use your Web browser to check out the aquatic plants section of the Krib.

HTML Translation by Erik Olson
Up to Fertilizer <- Plants <- The Krib This page was last updated 17 February 2002