As mentioned previously all Poly-Filters sold for usage in fresh or saltwater aquaria are preconditioned in synthetic saltwater ( Marine Enterprises , Crystal Sea, Bio Assay Formula) twice then rinsed in highly filtered de ionized water. We do this procedure for extraction ( of any residual monomers or copolymers ) and to add trace elements into Poly-Filter. All synthetic marine salt contains iron , in a 0.25 mg/L concentration, bound into the sodium chloride . All sodium chloride, producers list iron as the major impurity at 0.25 mg/L concentration. This iron is bound into the sodium chloride and cannot be detected using aquarium colorimetric test kits , instead either atomic absorption or Inductively coupled plasma lab testing is required. As such each Poly-Filter (R) contains 0.25 mg/L concentration of iron in it`s polymer structure. Freshwater aquarists use plant fertilizer , in most cases, because they believe it is required without testing for : nitrogen, phosphorus, sulfur, calcium, magnesium, potassium the major plant nutrients. Certainly every freshwater aquarium has an abundance of nitrogen, phosphorus, calcium and magnesium. Potassium and sulphur may require occasional supplementation. The micro nutrients : boron, cobalt, copper, manganese, molybdenum, zinc and iron are found in normal tapwater. Certainly the concentration added into Poly-Filter (R) from synthetic saltwater is higher than most aquatic plant supplement would contain. The result is Poly-Filter (R) would not adsorb the lower concentration of trace elements. If Poly-Filter (R) was adsorbing a iron concentration above 0.25 mg/L it would turn orange- red in color. A simple confirmation of the color being iron is to take a orange - red Poly-Filter (R) and soak it in a 3 - 5 % solution of Hydrochloric acid . The acid will elute out the adsorbed iron and the Poly-Filter (R) will turn back into white. Most well planted freshwater aquaria require very limited plant fertilizer supplementation. This is unless the aquarist wishes a extremely dense plant growth which requires metal halide lighting and CO 2 gas injection. Under these extreme cases iron and other trace elements may be required. However adding liquid plant fertilizer with iron is wasteful because aquatic plants cannot absorb nutrients through their stems or leaves. Aquatic plants have sealed stems and leaves and only absorb nutrients through the root system. The iron , trace elements must be adsorbed into the gravel then absorbed by plant roots. If iron is required we recommend using laterite clay, gravel for plants - it actually puts the iron into the gravel near plant roots. If freshwater aquarists use tapwater or filtered tapwater instead of R/O water ----- they do not need plant fertilizers it is redundant ! The tapwater and biological nitrification provides the nutrients for aquatic plants.
As a point of information : Before reef aquaria and R/O filtration aquatic plants didn`t require fertilizer supplementation. But after reef tanks started the pet manufacturers selling macro algae supplements for reef aquaria started telling everyone plants need supplements. Since today most coral tanks have limited micro, macro algae and SPS coral tanks have zero macro algae growth the algal supplements have migrated over into freshwater planted aquaria. Again these supplements were never designed for aquatic plants with root systems instead they were designed for macro algae which can adsorb these supplements directly out of the water.