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Re: Coal as a substrate

> Date: Sun, 30 Mar 1997 10:56:41 -0800
> From: babar100 at ix_netcom.com
> Subject: Re: Substrates (Coal?)
> There used to be a guy who used coal in his miniature furnished aquarium 
> exhibits at local open shows in the London area.  The rocks and the 
> gravel were composed entirely of coal.  It was an extremely attractive 
> set up.  The only problem I can think of is that the size of granule 
> would be difficult to control and it would have to be thoroughly washed. 
> Coal I believe is highly porous and so may make a good substrate.  All 
> this is IMO.
> David Brooks

I haven't used coal as a substrate but I can add something from my
background as a geologist.  You can buy black aquarium gravel and buy or
find a number of different kinds of black rocks that are probably more
suitable than coal. 

Coal is a natural trap for a variety of compounds that aren't typical in
aquarium materials.  Coal often contains substantial amounts of ferrous
sulfide (pyrite and marcasite) and sometimes gypsum - hydrous calcium
sulfate.  Both of these can react and/or dissolve in the tank.  Coal will
often also contain significant amounts of other metals - some of which
(like copper) are nice to have in trace concentrations but are toxic at 
higher levels. 

Also, coal is subject to oxidation (after all we do burn it for
fuel), and when oxidized under water the coal may turn grey and fall
apart.  It is also during oxidation that you might expect the trace metal
content to be released. 

This would not be a short-term effect.  Coal *should* be safe for 
short-term use - up to a few months anyway - but over a long term it 
could become a problem.  This is really a case where your mileage may 

Coal isn't particularly porous.  Very low grades of coal (lignite) might
be porous but in subbituminous grades and higher the original pores are
normally collapsed or filled with minerals and any porosity that exists is
in fractures.  Weathered coal - or coal that sits in your tank for a while
will contain some porosity caused by leaching of sulfides, gypsum, etc. 

Roger Miller