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I produce gravel for my aquarium by sieving and rinsing earth from my garden. After boiling and disinfecting it (with a solution against parasites) the rest of the organic material are composted or become invisible duff inside the gravel during the initial cycling of the aquarium. The only thing remaining visible are peat and other carbon particles of the size of the gravel which I currently need to remove mechanically.

Is there a way to remove them /shrink or reduce them to small pieces with a (chain of) chemical reaction(s) which

  • can be performed with a maximum of chemicals and equipment which can be bought at a supermarket or building supplies store
  • doesn't harm the fish after introducing them into the aquarium some weeks after the treatment - both through toxines and an instable PH value (more than +/- 0.5)
  • leaves the color and the size of the gravel untouched because my motivation to use custom gravel are mostly aesthetic and reducing the size would risk to block my undergravel filter (has a filter chamber with a separator for gravel of a certain size)

I thought about:

  • burning the particles, but that seems wasteful in terms of energy and requires some equipment (burner, fire proof permeable net, etc.)

I'm looking for a solution for some kg of 1mm to 3mm gravel in a 100 l aquarium filled with drinking water with a PH value of 7.2 to 8.

In case it's necessary to have more exact info what the particles in the gravel consist of, I'll ask a separate question how I can figure this out.

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It's easier to use flotation or froth flotation to separate out the less-dense peat (silt?) and carbon (I assume from the activated-carbon filter) than to use a chemical reaction..

Simple flotation would depend on making a dense solution (e.g. saturated Epsom salt, $\ce{MgSO4}$) and shaking the gravel mix in it... if the liquid's density is greater than the average density of silt with its entrained air, the gravel will sink and silt float, to be skimmed off. You can reuse the liquid for more separation.

Froth flotation relies on a surfactant wetting the silt and keeping it suspended while the gravel settles out. Try a bit of hand dish-washing detergent.

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  • $\begingroup$ I'll give it a try, thank you. I'm not sure if the minimal amount of dish-washing detergent I can use without risking to harm the fish (even after intensive rinsing) will have an effect. MgSO4 seems very affordable (< 1€/kg). $\endgroup$ – Karl Richter Jan 17 '17 at 0:17
  • $\begingroup$ Soap should also work in froth flotation -- more quickly biodegradable than most detergents. $\endgroup$ – DrMoishe Pippik Jan 17 '17 at 2:30
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    $\begingroup$ Feedback after try: I got fertilizer granulate "Compo Bittersalz", meaning bitter salt which is a synonym for MgSO4·7H2O according to the german Wikipedia and saturated Epsom salt, I think. It contained 16% MgO and 13% S which I assume became MgSO4 because it made much more bio mass float than tab water as expected. I added about 1 kg of the fertilizer to 1 l water until it no longer dissolved. Warming seemed to much of an effort because I couldn't maintain the temperatur during the separation anyway. I successfully removed about 95 % of bio mass which remained after intensive rinsing. $\endgroup$ – Karl Richter Jan 17 '17 at 21:48
  • $\begingroup$ Thank you! Can you confirm that the MgO and S became MgSO4 after putting it into the water? If not, would the effect be stronger if I'd get real MgSO4? $\endgroup$ – Karl Richter Jan 17 '17 at 22:04
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    $\begingroup$ You bought MgSO4... the chemical analysis is based on breakdown into other chemicals, e.g. MgO and S, to provide a percentage basis. $\endgroup$ – DrMoishe Pippik Jan 17 '17 at 22:19

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