If you follow these directions to the "letter" you will not have any problems.
DO NOT INCREASE THE DOSAGE FOR ANY REASON!!
The following treatment will kill algae and dissolve organic
matter (uneaten fish food, leaves, mulm etc). It is safe to use with pond plants
and fish, so do not remove these from the pond
Plan to do this treatment while you can stay right by the pond the entire
time. Should the fish start "gasping" at the surface, immediately add hydrogen
peroxide. If you walk away and the fish get in trouble you may come back and
find that they are all dead. I am very serious about this!! You MUST watch the
fish at all times!!! The fish will begin to move close to the surface as the
treatment progresses, and that is ok. They may occasionally come up to the
surface for air, and that is ok too.
I am talking about all of the fish gasping hard at the surface. If this
happens immediately add the hydrogen peroxide.
Add potassium permanganate at the rate of one TEAspoon per 1,000 gallons.
MEASURE VERY CAREFULLY. Do not get this in your eyes, or on your skin. Stand
down wind so there is no chance that you will breathe in the dust from it.
Potassium Permanganate is a very strong oxidizer. I'll give you a couple of
hints in addition here: