I thought the whole idea of cupramine was that it wasn't supposed to leave a copper residue.
Anyhow thanks for the comments. In case anyone's interested, I didn't use the tank in the end anyhow, had enough storage capacity without it for what I needed and had two empty tanks still in the queue ahead of this one so I was worried for nothing.
And, well, it's only water. I was just going to start dumping it down the drain if I needed more than 2 containers, I just didn't want to waste any if it was avoidable to do so.