Put some of your pond water in a jar and watch it and when it starts to have some color then you know the CS is not active.

https://srac.tamu.edu/serveFactSheet/66

Copper sulfate
(Various trade names)
Copper sulfate is a contact herbicide primarily used to
control algae. However, there are some species of algae (e.g.,
Pithophora) that copper does not control effectively. Copper
can interfere with gill functions and, if improperly used,
can be toxic to fish and zooplankton. In ponds culturing
young fish or fry, copper will often kill the zooplankton that
fish need for food and they will starve. Fish species such as
trout and koi are particularly sensitive to copper. However,
most fish kills that occur after copper sulfate treatment are
caused by a massive algae kill and the subsequent oxygen
depletion due to decomposition.
Copper sulfate is also formulated as a solution made
by dissolving the crystals in an acid. These acidified copper
solutions are registered for aquatic use and sold under
various trade names (e.g., Copper Cat®, EarthTec®, AgriTec®,
etc.).
The effectiveness and safety of copper sulfate are
determined by the alkalinity, hardness, pH, dissolved and
particulate matter, and temperature of the water. In water
with an alkalinity ≤ 50 ppm, the rate of copper sulfate
needed to control algae can be toxic to fish. Copper treatment
at water alkalinities of ≤ 20 ppm is extremely risky. In
high alkalinity (> 250 ppm) water, copper sulfate quickly
precipitates out and is not effective for algae control. The
toxicity of copper sulfate to fish increases as water temperature
increases. Avoid copper sulfate applications during hot
summer months. Copper is less effective in cold water (<
60 °F) and treatments should be delayed until temperatures
rise. (For additional information on treating with copper
see SRAC Publication No. 410, Calculating Treatments for
Ponds and Tanks.)

Last edited by ewest; 06/18/19 10:32 AM.