I won’t be going extreme on testing in any way but I do believe I could get some ph and dkh testers have you used or heard anything on the hanna testers iv seen em everywhereSo I buy my RODI from my local LFS.
Copper is bad for some fish as they are sensitive to it. And as someone else pointed out bad for inverts.
Some people go extreme for testing. But really nitrates phosphate and dKH are things you should test for. Mag and calcium if you have corals is helpful.