Yes, Flourish® is safe to use with shrimp. It is true that in large amounts copper can be toxic to aquatic animals, and invertebrates like shrimp and snails can be particularly sensitive. However, copper is needed in trace amounts by both plants and animals, including shrimp, which is why we include it in Flourish®, Flourish Trace™, and our other Trace products. The amount of copper in Flourish® is so small that you would have to massively overdose before you would begin to approach an amount of copper that is toxic to even the most sensitive animals. There is usually much more copper added by a water change using tap water than there is by dosing Flourish®.
The standard dose of Flourish® increases the copper concentration by about 0.002 mg/L (ppm). Most sources will report that copper starts to become stressful to shrimp at about 0.03 mg/L (ppm). That is more than a 15x overdose. Any product, if overdosed by 15x the recommended amount, would be stressful to shrimp. Copper is very rapidly utilized by plants, so there is no risk of attaining this level of overdose accidentally if dosing according to published instructions.