Potassium Sulfate

[Charles]

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.

[George Pontis]

According to Chuck Gadd's handy aquarium nutrient calculator, one would
add about 5 grams potassium sulfate per 100 litres just to get
potassium up to recommended levels for a planted tank.
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