_Maq_
Member
In private conversation, a member of this community asked me some questions about using plain silica sand as a substrate. In my country, silica sand ever was and still is a default substrate, so talking about its pros and cons seems rather redundant. Yet I have noticed that within this community, commercial substrates prevail to a degree that using silica sand looks like entering an unknown territory, and questions abound. Therefore, I decided to address some of the most common questions about silica sand publicly.
Size: Decades ago, hobbyists preferred large-sized gravels about 1 cm in diameter "to keep the substrate oxygenated". There's no reason to care for substrate oxygenation, largely because natural sediments are regularly oxygen-free and both plants and microbes are adapted to it. Indeed, moderately reducing (suboxic) conditions in the substrate are beneficial for bacteria driven nutrient cycling.
Silica particles should be round-shaped without sharp edges. Personally, after experimenting with various fractions, I prefer size in the range of 1 to 2 millimeters. Planting in such substrate is the easiest possible, bottom-feeding fish like it, and it's still large enough to let small particles of detritus to settle within (which is of some importance - see below).
Artificially coloured sands: Natural silica sand is white, light gray, or slightly brownish (due to impurities consisting mainly of iron oxides). Dark coloured natural sands are rare. Most of those commercially available are artifically coloured. Using them basically means that substrate is not made of silica but of its coating, which is usually some kind of plastic. They are mostly harmless but I have happened to find some with very negative adsorption qualities - not hydrophilic but lipophilic (hydrophobic). Tanks with such a substrate invariably ended badly.
Clarity: In my country, silica sands for pool filtration are preferred for their quality, purity, and favourable price. Still, it's recommended to wash them carefully before use. They may contain dusty particles so small that they even don't sedimentate and are the source of mineral bloom. Multiple water changes are necessary to dilute such a bloom to satisfactory clarity of water.
Chemical properties: Silica is almost insoluble and does not react with any compounds normally present in the tank. In this sense, it's inert. Fears of dissolving hydrated silica (silicid acid) into water are unsubstantiated. (Besides, dissolved silica in itself does not lead to diatom outbreaks. I fertilize with dissolved silica regularly, without issues.)
Physico-chemical properties: Silica sand is generally poor adsorbent for nutrients and other compounds. In newly established tank, we can take the concentration of nutrients in the water column and within substrate for equal. This quickly changes, though, once the tank begins to "live". Tiny particles of detritus penetrate into the substrate, and plants' roots die-off there, too. Detritus is an excellent adsorbent for phosphorus and transition metals (Fe, Mn, Zn, Cu, Ni). Silica sand enriched with detritus is also fully capable to host complete microbial community, including nitrifying microbes. (I've performed some experiments which proved it without doubt.) Indeed, I maintain that within reasonable limits of livestock population, any and all filters with their fabulous media are redundant for biofiltration. Within substrate, microbes take on natural zonal distribution in accord with redox (from fully oxidized to anoxic). Microbial community structured in this way is complete (unlike within filters) and ensures complete cycling of all compounds, organic and mineral. Plants do not expect and require anything else.
Enriching silica sand: I never enrich substrate with nutrients and have serious doubts whether this is recommendable. However, some additives can strenghten substrate's adsorbing ability. Among those I've tested several clays (purified ones available commercially in powder form), zeolites, powdered ferric (III) oxide, activated carbon, and peat.
Clays form significant part of most soils. They feature huge cation exchange capacity and, in theory, can act as buffers, stabilizing levels of metallic cations as well as protons (H+, which is acidity). Heavily protonated clays can exchange protons for metallic cations, which is the principle of action of commercially available acidifying and softening "Japanese" substrates.
Zeolites are powerful adsorbents but their modes of action strongly depend on the given kind of zeolite. Clinoptilolite is commonly used for its strong exchange affinity for ammonium (NH4+, cat litter) and can help keep ammonium within substrate, close to plants' roots and harmless for fish. In theory.
Powdered ferric oxide works as a long-term source of iron, and - more importantly - as a powerful adsorbent of phosphorus and transition metals. There's nothing unnatural about this additive as ferric oxide is present in almost all soils and microbes as well as plants' roots are well equipped to acquire nutrients from this source. (On the contrary, if detritus gets caught within filter, it adsorbs iron in the form of ferric (hydr)oxide, which then adsorbs phosphorus and micronutrients. However, since we keep our filters strongly oxidized, these nutrients seldom get reduced and are trapped in the filter forever.)
Apart from mentioned above, ferric oxide helps preventing hydrogen sulphide problems because these two readily react into harmless iron sulfide (black precipitate).
Activated carbon in itself is inert but it provides unparalleled (incl. expensive commercial filtration media) colonization area for microbes and adsorption capacity for organic compounds. My experience with this additive is negative. Activated carbon is lighter than silica, within time it tends to accumulate on the surface and algae seem to be crazy to settle and prosper on it.
Peat mostly serves for acidification and as a source of humic substances. It does not decay, have no fears.
To summarize my experience with above mentioned additives, I have to admit that my results failed to produce manifest effects. That's not to say that they're bad. There's no reason to doubt they work as theory says. Yet in my opinion, it would require large-scaled long-term experiments to demonstrate tangible effects and to gain reliable data how to use these additives with optimum results. That's obviously beyond my amateurish possibilities. As a result, for now, I stick to plain silica sand. It works and it's the safest bet.
Size: Decades ago, hobbyists preferred large-sized gravels about 1 cm in diameter "to keep the substrate oxygenated". There's no reason to care for substrate oxygenation, largely because natural sediments are regularly oxygen-free and both plants and microbes are adapted to it. Indeed, moderately reducing (suboxic) conditions in the substrate are beneficial for bacteria driven nutrient cycling.
Silica particles should be round-shaped without sharp edges. Personally, after experimenting with various fractions, I prefer size in the range of 1 to 2 millimeters. Planting in such substrate is the easiest possible, bottom-feeding fish like it, and it's still large enough to let small particles of detritus to settle within (which is of some importance - see below).
Artificially coloured sands: Natural silica sand is white, light gray, or slightly brownish (due to impurities consisting mainly of iron oxides). Dark coloured natural sands are rare. Most of those commercially available are artifically coloured. Using them basically means that substrate is not made of silica but of its coating, which is usually some kind of plastic. They are mostly harmless but I have happened to find some with very negative adsorption qualities - not hydrophilic but lipophilic (hydrophobic). Tanks with such a substrate invariably ended badly.
Clarity: In my country, silica sands for pool filtration are preferred for their quality, purity, and favourable price. Still, it's recommended to wash them carefully before use. They may contain dusty particles so small that they even don't sedimentate and are the source of mineral bloom. Multiple water changes are necessary to dilute such a bloom to satisfactory clarity of water.
Chemical properties: Silica is almost insoluble and does not react with any compounds normally present in the tank. In this sense, it's inert. Fears of dissolving hydrated silica (silicid acid) into water are unsubstantiated. (Besides, dissolved silica in itself does not lead to diatom outbreaks. I fertilize with dissolved silica regularly, without issues.)
Physico-chemical properties: Silica sand is generally poor adsorbent for nutrients and other compounds. In newly established tank, we can take the concentration of nutrients in the water column and within substrate for equal. This quickly changes, though, once the tank begins to "live". Tiny particles of detritus penetrate into the substrate, and plants' roots die-off there, too. Detritus is an excellent adsorbent for phosphorus and transition metals (Fe, Mn, Zn, Cu, Ni). Silica sand enriched with detritus is also fully capable to host complete microbial community, including nitrifying microbes. (I've performed some experiments which proved it without doubt.) Indeed, I maintain that within reasonable limits of livestock population, any and all filters with their fabulous media are redundant for biofiltration. Within substrate, microbes take on natural zonal distribution in accord with redox (from fully oxidized to anoxic). Microbial community structured in this way is complete (unlike within filters) and ensures complete cycling of all compounds, organic and mineral. Plants do not expect and require anything else.
Enriching silica sand: I never enrich substrate with nutrients and have serious doubts whether this is recommendable. However, some additives can strenghten substrate's adsorbing ability. Among those I've tested several clays (purified ones available commercially in powder form), zeolites, powdered ferric (III) oxide, activated carbon, and peat.
Clays form significant part of most soils. They feature huge cation exchange capacity and, in theory, can act as buffers, stabilizing levels of metallic cations as well as protons (H+, which is acidity). Heavily protonated clays can exchange protons for metallic cations, which is the principle of action of commercially available acidifying and softening "Japanese" substrates.
Zeolites are powerful adsorbents but their modes of action strongly depend on the given kind of zeolite. Clinoptilolite is commonly used for its strong exchange affinity for ammonium (NH4+, cat litter) and can help keep ammonium within substrate, close to plants' roots and harmless for fish. In theory.
Powdered ferric oxide works as a long-term source of iron, and - more importantly - as a powerful adsorbent of phosphorus and transition metals. There's nothing unnatural about this additive as ferric oxide is present in almost all soils and microbes as well as plants' roots are well equipped to acquire nutrients from this source. (On the contrary, if detritus gets caught within filter, it adsorbs iron in the form of ferric (hydr)oxide, which then adsorbs phosphorus and micronutrients. However, since we keep our filters strongly oxidized, these nutrients seldom get reduced and are trapped in the filter forever.)
Apart from mentioned above, ferric oxide helps preventing hydrogen sulphide problems because these two readily react into harmless iron sulfide (black precipitate).
Activated carbon in itself is inert but it provides unparalleled (incl. expensive commercial filtration media) colonization area for microbes and adsorption capacity for organic compounds. My experience with this additive is negative. Activated carbon is lighter than silica, within time it tends to accumulate on the surface and algae seem to be crazy to settle and prosper on it.
Peat mostly serves for acidification and as a source of humic substances. It does not decay, have no fears.
To summarize my experience with above mentioned additives, I have to admit that my results failed to produce manifest effects. That's not to say that they're bad. There's no reason to doubt they work as theory says. Yet in my opinion, it would require large-scaled long-term experiments to demonstrate tangible effects and to gain reliable data how to use these additives with optimum results. That's obviously beyond my amateurish possibilities. As a result, for now, I stick to plain silica sand. It works and it's the safest bet.
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. I have given silica a lot of thought and have initially planned to use it in my next tank
