It depends on the age and or quality of the silicone i think. I have a 20 gallon tank that i use as a fo/qt tank that was exposed to copper over 10 years ago. It has all new equipment and new rock and sand since then. It still to this day will grow almost no algae in the tank. If i really let it get super nasty before a do a change (it currently houses a 24" golden tail eel), it grows a black/brown dust type of algae, but never anything else. I have never tested the levels, because i know what it was exposed to, and it is fo/qt, so it doesn't matter the levels. I don't know if this is typical or not, but it's not worth the risk IMO. I would set it up with water only for a week or two, and test again before putting more rock in there. It don't take much copper to kill corals.