|Here is an excellent case study on PMDD. It is a do-it-yourself (DIY) fertilizer mix, and when used with other common recommendations can yield a reduced algae environment. PMDD is more of a method of fertilizing then a recipe for fertilizer. Yes the basic mix is given, but the intention is that you will adjust it to fit your particular set of conditions. Dosage rates and the ratio of the ingredients can all be varied based on observation and measurement to obtain a more favorable condition in the tank. Here is a newer update, or a PMDD companion article to the PMDD article linked above. The companion article lists sources of the nutrients and is basically a "how to." The two big considerations are Fe levels and NO3 levels. You adjust the dosage based on a target of 0.1ppm iron and you adjust the potassium in the mix by monitoring the NO3 level. Ideally you will minimize the Nitrate level, not eliminate it, just keep it low.
I bought my Potassium Sulfate (K2SO4) and Potassium Nitrate (KNO3), along with the trace element mix at Homegrown Hydroponics and the Magnesium Sulfate was from the local supermarket, along with the distilled water. Again the things that makes this so great is you can tweak it to fit your particular tank. For example if you have a low fish load you can increase the proportion of the KNO3 to prevent a nitrogen limited condition. Or in the opposite situation and you have to many fish you can increase the amount of K2SO4 and decrease or eliminate KNO3. It offers quite a lot of flexibility, and I will have to say the plants have done better on this then any other plant food combination. Another benefit of having separate components to mix together is that you can dose anything you may want separately. For example I add about 1/8 of a teaspoon of K2SO4 per 10 gallons of water during water changes. This keeps my Potassium levels up so K is not my limiting factor.
The target concentration of K is 10 mg/l or ppm. Also a target or minimum of 40ppm Calcium and 10 ppm magnesium are also numbers to maintain. The calcium and magnesium along with a KH of 3°+ are all maintained by my tap water supplier so I do not have to worry there.
There are a wealth of commercially available mixes in the stores. Kent has a good one, and Seachem's Flourish is said to work well, or the Dupla regime is excellent along with Tropica's Master Grow. The Dupla and Tropica products are available at Monolith Marine Monsters. Dupla is also available at Pet Warehouse and J. P. Burleson.
Be careful though some of the liquid fertilizers are just trace mixes. They lack K and can lead to a surplus of Fe and therefore algae problems. Some mixes also may have portions of N & P which are normally two elements that do not need to be added. This is the main reason terrestrial fertilizers are not recommended for use in aquariums. Some mixes also have little Iron.
The Dupla regime and TMG have K. But the Dupla approach needs to be followed with all the components to assure the correct dosage of K. Dupla has daily drops along with a tablet to dissolve in the replacement water. These tablets contain the elements in fertilizers, like Potassium that can be dosed without large concerns of algae problems and also components that will last for a couple weeks.
Another area of a fertilizer plan are Substrate additives. Again there are many on the market and they all work to a varying degree. Laterite, an old tropical clay that is rich in iron and silica is one of the best I have found to date. There are numerous claims to its success, but the repeatable results make it a good choice, and what I will always recommend to the first time Aquatic gardener. I have used the Thiel product as it is a lot cheaper then the Dupla, but I hear the Dupla one is a little better. I also just recently heard Thiel is no longer selling Laterite. Next time I buy laterite I may try Dupla's. My latest plant tank is an experiment with a soil substrate. The 20 gal experimental tank has it's one page. There are other commercial substrate additives on the market like Terrapur, Substrate Gold, Sechem has one, Amano has some, Tetra too. There are a lot others out there, but for now Laterite is my main recommendation. I have not tried the Substrate Gold, but it is a enhanced laterite and in theory should work well. A note on the Dupla approach. Many people badmouth Dupla due to their cost. Granted Dupla products are a little high, and a lot of that has to do with import taxes. But cost aside their products work VERY well! And if you factor it all together the cost is not that high. Also their fertilizers are not to bad price wise where their hardware is a little off base. Again excellent quality and functionality, but expensive.
Other DIY approaches in substrate additives call for varying mixes including anything from topsoil, subsoils, vermiculite, Kitty litter, peat moss, pottery clay, earthworm castings, manure, compost, sands, gravels, etc. Another approach often overlooked is to plant in pots. Some people wish to cater their soils to their plants specific needs and mix and match in small pots imbedded in a gravel base. This can work very well especially if a few plants want a rich organic substrate. That way you do not have to load up the tank full of organics just small zones for individual plants. Please read through Steve Pushak's site on Substrate articles.
Lastly with regards to substrate fertilization is adding solid fertilizers to the substrate. As indicated by plant growth, it is a somewhat common practice in some methods to push fertilizers into the gravel on a regular basis. Some people add laterite balls every year, roll balls of clay(Laterite or pottery clay) with terrestrial plant food, just stuff solid plant spikes, injecting a slurry of clay, and other forms of commercial aquarium ready preparations. These additions are usually made when the gardener feels the plants need it, and are stuffed deep into the substrate to avoid leaching into the water column.
One other area that has a major impact on plant growth is the addition of CO2. This is sometimes called a fertilizer, well I guess that is appropriate, but I will discuss it in another section.
Last updated 07/23/02 JCD