mechanics of blackout

[Brown_D at pcfnotes1_wustl.edu]

On my previous trips out of town, I premeasured the fish food and put each
day's feedings into a separate small container, so my fish caretaker would
just open one for each tank each day I was gone.  This time she followed me
around the tanks one evening, and took notes, and did the feedings
herself.....but I came home to very overfed tanks (floating and sinking
excess food in 3 of 4 tanks) and a new horrid almost clear greyish slime on
everything in one tank.  I am not sure if it is bacterial or algal slime,
even, because it's not green.   I vacuumed the gravel, rinsed the filter
media, and did a 50% water change, and things looked better this morning,
but still not good.  I would like to try a blackout, although if it's
bacterial it may not be affected by the blackout....but maybe the thick
black brushy algae will go away even if the greyish stuff doesn't.

So how do you do a blackout?

I put some plastic garbage bags over the tank, so they overlap it loosely,
but did not tape or otherwise seal the edges (I don't want to suffocate my
two goldfish in their now-slimy tank).  I have unplugged the lights.  Is
that sufficient for a blackout?  And is it ok to lift the plastic to check
on the fish--one was looking a little punky with his fins not as erect as
usual--or will even a few minutes of dim light now and then spoil the
effect of the blackout?

Thanks in advance for you advice,

Diane Brown