I am in the process of setting up a 75 gallon reef tank. I am using one of my old 30 gallon freshwater tanks as part of my DIY sump. I was not the first owner of this tank. It was setup as my discus tank for about 3 years until I recently upgraded it to a 55 so I could use the 30 for my sump. I recently read that if the silicone is green it means the tank had been previously treated with copper, while I have never treated the tank with copper I am unsure if the previous owner did or not. Only the silicone on the bottom piece has a green tint to it the sides are all clear. I filled the tank with freshwater about 5 days ago and it reads 0 for copper. How long does it take for copper to read positive? Is there any way to clean the copper out? My baffles are already installed in this tank and would rather not have to start over but the last thing I want is to fill my 75 with pricey coral and inverts only to have it all die.

Copper is brought into water when the tank is acid (ph<7.0). Give your tank a ph of 5 or below and test for copper after a few days. I suspect you will have none. If your new inverts are very expensive and/or rare I would just buy a new tank and save your old one for FW.

Yeah, honestly I would rather not risk it over a $100 dollar coral frag.

i hope it is free.