JBL Cu ProAqua Copper Test 50 measurements, 0.1-1.6 mg/l. Copper is a heavy metal which on the one hand is indispensable as a trace element for plant and animal organisms, but on the other hand in excessive concentrations has a devastating effect as a cell poison. Copper usually gets into aquariums through heavy metals in tap water or metal bodies in the water, and into ponds through gutters containing copper. Copper can be bound by good water conditioners. Copper can be used specifically to combat fish diseases.