Coptrol targets algae, is more effective, far less harmful than bluestone.
Nevertheless there is always a risk as copper demand in water varies widely. We know that trout and koi carp are particularly sensitive to copper demand and it is possible Coptrol may be toxic to Australian native fish and crustacea even in hard water.
Coptrol use in fish ponds and aquaculture should be approached with caution and only use when algae infestation poses a serious threat to fish or crustacea.
If Coptrol is used there are some strict rules to follow:
Do not use Coptrol if the Calcium hardness of the water is less than 50 ppm. Below a level of 50 ppm of hardness, copper in a toxic form may be liberated thus harming fish and crustacea. Water hardness may be tested by using a commercial hardness test kit, or having a small sample (about 250 mL) tested in any NATA registered laboratory.
Do not use Coptrol if the water temperature is lower than 16 degrees C (61 deg F) . Below 16°C (61°F) spatiation may occur where copper can separate from the chelating agent, potentially being harmful to fish and crustacea.
Do not exceed the recommended dose for the size of the pond.
Do TEST a few fish in a bucket or other suitable receptacle before treating the pond
Do remember to treat the shoreline first to avoid trapping fish in shallows
In ponds containing fish treat only 1/3rd of the area at a time to avoid oxygen depletion of the water caused by the decaying dead algae. Allow 10-15 days for oxygen levels to recover before continuing treatment. Lack of oxygen may cause fish to suffocate.
“Copper demand” explained
Copper has been used in balancing pond and water environments since the inception of the aquaculture industry. However, the concentrations of copper are critical and the "copper demand" of water varies widely. It is essential that this demand be known, as copper will combine with carbonate ions in water (which give the water its "hardness") to form insoluble copper carbonate. Thus, the copper goes out of the system and more must be applied to achieve the correct balance. This is often a trial and error process, which is costly in terms of time and money.
Copper in its ionic forms, e.g. copper sulphate, is potentially hazardous to fish and presents some risk even when great care is taken in treating algae‑infested waters that contain fish. However, Coptrol (a chelated copper compound) is highly specific to algae and has been shown to present a lower minimum hazard to fish when used in accordance with the label directions.
Effects of copper on fish
A large number of studies have been conducted on the toxicity of copper to fish, whether the copper is naturally occurring or introduced.
The label cautions against the treatment of waters containing trout and other species where water hardness is lower than 50 ppm.
Moore (1984) describes naturally occurring copper in the ionic form as potentially more toxic to fish than any other heavy metal except mercury. However, he notes that copper is much less toxic to fish in waters with a high complexing capacity, such as salt water. He also quotes data to confirm that copper is significantly less toxic to trout in hard waters.
Moore (1984), Stiff (1971) and Pagenkopf (1974) are in general agreement that ionic copper (Cu) and ionised hydroxides (such as CuOH) are the forms of copper most toxic to fish. Coptrol contains copper only in the chelated form, in contrast to copper sulphate which presents copper only in the ionic form.
Stiff (1971) noted that "natural" complexing of copper ions with glycine and humic substances caused some reduction in copper toxicity.
Sprague in 1968 suggested that the chelating agent NTA (nitrilo-triacetic acid) could be used as an anti‑pollutant to protect fish from copper poisoning.
Boyd (1979) reported that chelated copper algaecides are often recommended for application to soft water ponds to prevent copper poisoning in fish. He further noted that "The toxicity of the chelated (copper) compound to fish is less than that of an equal concentration of copper in copper sulphate, so that species with a high susceptibility to copper are not harmed in waters of low alkalinity".
Moore (1984) reported, "In copper polluted fresh waters, a maximum concentration in muscle tissue seldom exceeds 1 mg/kg wet weight. Because muscle residues are generally low; copper does not pose a threat to most fisheries, even those in polluted waters.