I think youre right - .2 is the 'minimum therapeutic dose'. According to their website: There is more than a four-fold concentration gap between the minimal therapeutic dose (0.2 mg/L) and the toxic dose (0.8 mg/L). Cupramine™ is easily removable with chemical filtration. It is highly effective and safe in freshwater as well as marine water. If the bottle has a dropper cap, use 20 drops (1 mL) per 40 L (10.5 US gallons) the first day, wait 48 hours, then repeat. On non-dropper caps, each inner ring is 1 mL. In freshwater use half dose. Final copper concentration is 0.5 mg/L (0.25 mg/L in freshwater). Leave at this concentration for 14 days. Do not redose without testing (MultiTest™ Copper). If tank has ever been treated with an ionic copper (e.g. copper chloride, sulfate or citrate), test copper level after initial dosing. Although most fish tolerate Cupramine™ to 0.8 mg/L, it is not advisable to exceed 0.6 mg/L copper.