I decided to add some to a tank, figured out what I thought would be
an appropriate amount. For about 144 liters of water I thought 0.6
grams would be right, a chunk I picked up weighed 0.5, so I went with
that.

Reading Christel Kasselmann's book, she says to use 8 grams per
hundred liters to kill cyanobacteria. I guess I was well within the
safe limits.