What brand test kit are you using? There are 2 type of copper medication (chelated copper and non-chelated copper) and each uses a specific type of test kit. CopperSafe is chelated copper and an API test kit works properly with it.
The crushed coral and base rock absorbs the copper. It will reach the same levels as the water it's in. It will take a long, long time for it to leech out back into your water. You can do a 100% water change and it might test 0 ppm
copper, but it will still be in the crush coral and base rock and slowly leech out. It just takes a little to kill an invert. The safest option at this point is throw it away or never add any inverts.