Originally known as Poor Man's Dupla Drops it was created by Paul Sears and Kevin Conlin in 1996 as a method to supply the plants in the aquarium with all the necessary nutrients, but to restrict phosphate as a means to control algae. Phosphates must be restricted, but not eliminated, otherwise the plants will start to die. Originally the PMDD formula had no phosphates in it as it was presumed that enough would be available in the tank from the fish. In recent years though it has been shown that potassium phosphate is also required in the formula as not enough to supply the plant's needs comes from the fish. From this dosing method came Estimative Index (EI) which doses higher levels of nutrients, but resets the tank's levels with large weekly water changes and the Perpetual Preservation System (PPS-Pro) which is very similar to PMDD but with phosphate added and which tries to match nutrient dosing with plant uptake so reducing water change frequency. There are also many other variations of PMDD which are used very successfully around the world by experienced aquatic plant keepers. Even many hydroponics systems use the PMDD formula with great success.

The PMDD solution is added everyday so as to keep levels as stable as possible and help prevent any of the nutrients running out. Dosing levels have to be acquired by testing for iron and nitrates over a period of weeks. This is one of the major pitfalls of this system because of the difficulty in obtaining accurate test results.

1 Teaspoon  Potassium Nitrate
2 Teaspoons Potassium Sulphate
2.5 Tablespoons Magnesium Sulphate (Epsom salts)
1 Tablespoon Chelated Trace Element Mix (7% Fe, 1.3% B, 2% Mn, 0.06% Mo, 0.4% Zn, 0.1% Cu, EDTA, DTPA)
300 ml  distilled H2O

On the Krib website there is a formulation for Conlin and Sears' latest PMDD formula which is slightly different.
1 Tablespoon  Potassium Nitrate
2 Tablespoons Potassium Sulphate
1 Tablespoon Magnesium Sulphate (Epsom salts)
1 Tablespoon Chelated Trace Element Mix (7% Fe, 1.3% B, 2% Mn, 0.06% Mo, 0.4% Zn, 0.1% Cu, EDTA, DTPA)
500 ml  distilled H2O

Many people have seen a need for PO4 to be added to the original PMDD formulation so I have come up with a formula that is based on the latest formula found on the Krib website but with the addition of potassium phosphate. In the original formulations all of the ingredients are mixed into a single solution, but this can cause problems due to the potassium phosphate which will react with any free iron. It is therefore easiest to make up two separate solutions for macro's and micro's. But it is possible to mix all the ingredients together by keeping the iron chelated. See my 'All In One' solution for more details.

Macros
25g Potassium Nitrate
2.2g Potassium Phosphate (monobasic)
11g Potassium Sulphate
20g Magnesium Sulphate Heptahydrate (Epsom Salts)
500 ml distilled water

Micros
15g Chelated Trace Element Mix (7% Fe, 1.3% B, 2% Mn, 0.06% Mo, 0.4% Zn, 0.1% Cu)
250 ml distilled water
0.5 ml Normal Hydrochloric Acid

The level of dosing is dependant on your plant stocking level. The amounts below are designed for a fully planted tank so if you are 50% planted then you can half the amounts. These are the amounts that I use on my tank and that I know work for me. They are both dosed on a daily basis.

2 ml of Macros solution per 10 gallons (US) or 40 litres
1 ml of Micros solution per 10 gallons (US) or 40 litres

For those wishing to know each dosing will add:

1.5 ppm NO3
0.15 ppm PO4
1.5 ppm K
0.2 ppm Mg

It is best to dose at the same time each day, preferably just before lights on. I dose in the morning before I leave for work.

There are no set guidelines for doing water changes as the levels of dosing are quite small, but I'm very keen on doing weekly water changes. The choice is up to you but 30 to 50% every week or two is a good place to start.

Due to a lean water column and slower plant growth CO2 levels don't need to be run as high as 30ppm, but they do need to be stable day after day. I'm currently running at about ~25ppm. It is probably best if you have pressurised CO2 as yeast based CO2 is just not reliable enough.

If you prefer to use Tropica Plant Nutrition or Seachem Flourish then just add at the same amount as the trace element mix.

Have high levels of nitrate and phosphate in your tap water? Then you can lower the amounts in the macros solution. Will require testing and a bit of trial and error to get the right levels.

Green Spot Algae seems to be one of the main problems with using a lean PO4 dosing regime. A little bit of experimentation with the PO4 amounts I've found will normally fix the problem.

You will need to adjust your lighting period by how much light you have above the tank. The more light you have the shorter the light period you require. Don't go below 6 hours per day or above 12 hours. Between 8 and 10 hours is fairly normal but if you run above 2.5wpg of T5 you may need to reduce the light period to below 8 hours.

Practical PMDD Information (Sources and Doses)

Control of Algae in Planted Aquaria

Paul and Meg Sears' Web Page - Planted Aquaria