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Re: Aquatic Plants Digest V4 #498

I wrote:

> 1)  With the exception of some initial settling there should be no
> long-term compaction.  Sand doesn't compact anymore than gravel.  If you
> tried contouring the substrate and didn't use something to keep the sand
> in place then the sand will gradually shift from areas with thick
> substrate to areas that initially had thin substrate.  Eventually you'll
> have a flat substrate.<<

And Robert H. responded:

> I have to disagree with you Roger. I have seen sand compacted. Someone gave
> me a 20 gallon tank with sand still in it and enough water still in it to
> cover the sand. The tank was a year old and the sand was so compacted I
> couldnt poke my finger through it. I have never seen gravel do this. I also
> had a problem with a sand and gravel substrate last year, but I suppose
> there could have been other factors involved. When there is a foul smell,
> black roots, and fish dieing, I dont see how it can be a good thing!

The clean, well-sorted (poorly graded) sands we usually find for use in
aquariums forms a bed with about 40-50% pore space when its' first put
down.  Over a period of weeks or months thereafter it will settle to the
point where it contains 35-40% pore space.  Once it reaches that state no
further compaction will generally happen.  It can stay in that state for
millions of years and support the weight of (at least) hundreds of feet of
other sediments sitting on top of it without further compaction.

Small gravel acts much the same way.

After it has settled it is firm (much like wet beach sand at low tide),
but can be easily loosened. Plant roots can still penetrate it and fish
can still stir it about.

Sand by itself doesn't contain anything that will cause nasty anaerobic
conditions.  That is caused by bacteria consuming organic matter.  It can
happen in sand or in gravel.  I only see it in my tanks under and around
driftwood where it can happen in sand or in gravel.
> One thing about sand I am curious about, sand that is mostly silica, would
> that have any affect on diatomes?

It could, I suppose.  But then, the glass is made of silica too, so it
might be hard to sort out the blame.  Gravel (most or all I've ever seen
for fresh water tanks, anyway) also contains a lot of silica.  

> I also found that most of the sand at
> least what is available around here is actually made of crushed coral and
> clamshell, and even mixed with silica sand, even if it makes no mention of
> it on the bag. I called the manufacturer of a common blasting sand that is
> sold in many aquarium stores around here, (the name escapes me) and was told
> that almost all sand sold commercially for blasting or pools actually
> contains at least some seashell. (he didnt mention "play" sand) This along
> with problems I experienced made me turn away from sand.

That's a regional thing.  Around here you'd have to look long and hard for
a source of sand with a significant amount of shell material in it.

Roger Miller