Why change water in planted tanks?

> From: richardm at cd_com (Richard Masoner)
> Date: Fri, 10 Jan 1997 10:33:18 -0600 (CST)
> I know one of the requirements of keeping a heavily planted tank is
> frequent water changes (which I do semi-faithfully), but what is the
> reason?  I've always thought that water changes were to keep the end
> product of the nitrogen cycle, nitrates, diluted.  But in a heavily
> planted tank, there shouldn't be an excess of nitrogen.
> So are there are pollutants that we're getting rid of here in planted
> tanks?
> Richard Masoner
> in windblown sub-freezing snowy Champaign Illinois
> [ -40 degree (C & F) windchill. Brrrr! ]

Just think of it as a box, with stuff that goes in (fish food, 
trace elements, water to make up for evaporation, etc) and 
stuff that comes out (pruned plants, vacuumed litter, stuff
that you remove when you clean the filter). What are the odds
that the composition of what goes in  is exactly the same as
the composition of what comes out?  Whatever differs between 
these will accumulate over time, and eventually some form of 
algae that likes that accumulated stuff will take over your 
tank. I speak from experience!

Glen Osterhout