Heating Coils/Substrates

     I have used fine sand (as opposed to gravel) as a substrate for years. 
     Here's a summary of advantages:
     1. Inert.  Does not affect pH or (apparently) leach anything into the 
     2.  Cheap.  Less than $5 for a hundred pounds at the hardware store.
     3.  Attractive.  Light brown flecked with dark bits and not blinding   
     4.  Accomodating.  All plants I've grown have developed ENORMOUS root 
     systems very quickly.  Corydoras love to root around in it.
     5. Convenient.  Crud (uneaten food, fish poop, etc.) sits on top and 
     is easily vacuumed off.
     1.  Because of compromised water movement, may become anaerobic.  One 
     time I set up a tank with a bit of a nitrogen source (sterilized 
     composted cow manure) in the substrate.  Despite lots of plants, H2S 
     was produced, which fed a never-ending bloom of sulfur-metabolizing 
     bacteria and caused permanently cloudy water.  After many gyrations I 
     finally gave up and tore down this tank and started over.  
     2.  Right now the substrate in my tank is almost a solid mass of 
     roots.  Impossible to pull anything out without disrupting the entire 
     tank.  Large Echinodorus have roots from one end of the tank to the 
     other.  Growth slows down after a while except in the very well rooted 
     plants, which continue to flourish.  I have a some gorgeous sword 
     plants (E. osiris. E. bleheri,  E. martii, E. uruguayensis) which 
     still produce new leaves every few days.  It seems that new plants 
     don't adapt well.  Inflexibility
     3.  Possible long term instability.
     Like many others, I'd like to address this last point. and try heating 
     coils at some point.  It would seem that the small grain size of sand 
     would inhibit water movement, and simply produce a conductive rather 
     than a convective situation.   Would it be possible to induce slow 
     flow through fine sand?  Would a switch to larger grain size gravel be 
     necessary for heating coils to work properly?
     What do y'all think?
     Bob Hoesch
     National Fish & Wildlife Forensics Laboratory
     Ashland, OR
     Bob_Hoesch at fws_gov