Tank turned from clear to completely milky in <2 hrs

[Shane Graber <SGraber at sauder_com>]

This is going to sound like an incredibly n00bish question but it's never
happened in my 10+ years of keeping freshwater and marine tanks. Here's the
scenario:

I have a 10 gal freshwater planted tank @ work based on soil substrate per
Diana Walstad's book Ecology of the Planted Aquarium.  Yesterday @ 5 PM, I
did a 5  gal waterchange on the tank using dechlorinated tap water.
Everything looked fine when I left work as well as when I came into work
today.  Around 11 AM this morning, the two native darters (the only fish in
the tank presently) started gasping and swimming around the tank --
basically freaking out.  They then started laying over on their sides on the
bottom of the tank and not moving.  From the start of them 'freaking out' to
the time that they stopped moving in the tank was < 10 minutes.  I quickly
grabbed a bucket, made fresh dechlorinated tap water and quickly threw them
in there as I thought something was up w/ the tank.  W/in 5-10 minutes of
them being in there they started acting normal again.  As of 1 PM they're
back to normal.  I left them in the bucket as I wanted to do some water
testing on the tank before I put them back in.  Around noon (1 hr after this
all started) the tank started turning cloudy.  I didn't think much of it
when I left for lunch but then I came back around 1 PM the tank looked like
someone poured an entire gallon of milk in the tank.  I checked for ammonia
and nitrate and it came up zero on both tests.  I'm at a complete loss as to
what happened.  

Any of you guys see this before???

Shane