I hope this helps with the numbers.
Copper in Fresh & Marine Waters. Levels of copper in fresh and salt water have been found to be generally low. Studies of raw, untreated fresh water in the US have shown copper contents ranging from 0.001 milligrams per liter to 0.28 milligrams per liter. The mean was 0.015 milligrams per liter. In open oceans, copper levels ranged from 0.1 milligrams per liter to 0.39 milligrams per liter, with an average of 0.18 milligrams per liter. These figures show how copper is effective in small quantities.
Copper in Fish and Aquatic Life. The normal level of copper in whole fish tissue is on the order of one to two parts per million. The amount of copper and other trace minerals in the growth and development of crustaceans (shellfish) and mollusks are minute in quantity measuring less than one part per ten million.
Copper in Aquatic Plants. Aquatic plants are reliant on copper, which plays an important role in photosynthesis and respiration. Plants get copper that is dissolved in the water, as well as copper that is present in particles, sediment, and in food.
I pulled this data from a number of different studies on the net. IMHO, hobbyists should not add copper to tanks with inverts. There is more than enough copper in tap water and fish food. I would hate to think of any serious hobbyist intentionally posioning their animals.
Smoke 'em if you got 'em.
20g Tall Low Tech Planted