You can test for copper, but if that was in the water column before, your corals would have been dead very quickly before this, and the copper will leach into the LR, and come out very slowly. I don't think it's copper. I think that you have an abundance of nutrients that algae can live on. If it was bad before, all you do when you kill algae is release the nutrients back into the water column, and living algae are made up of the nutrients we don't typically like high levels of. What you may have done when you changed all the water is give the algae fresh nutrients to grow with that they may have been rate limited on before that, such as iron, so you just gave them something they need to grow.
That being said, I'm just guessing at this point since I can't see your tank in person nor do I have enough info to go on. What do you mean all your tests are fine? What test kit are you using?
That being said, I'm just guessing at this point since I can't see your tank in person nor do I have enough info to go on. What do you mean all your tests are fine? What test kit are you using?

