Should I treat main tank with Cupramine? If not, how will the issue ever stop? Do you know of it turns anything in the tank green like the other brands?
Well, there are two big issues when treating a DT with Cupramine:
#1 Your rocks/sand will absorb a lot of it, so you'll have to keep testing your copper level and adding more Cupramine on an almost daily basis. This problem subsides after your rocks have absorbed all they possible can, but expect your copper level to keep dropping below minimum treatment levels in the very beginning.
#2 Because your rocks/sand will absorb a lot of copper, you'll never be able to safely use them in the same tank as corals/inverts ever again. Some people will later soak the rock is muriatic acid to get the copper out, but I'd always be paranoid about some copper leaching back into the water. Even a tiny amount of copper is deadly for most corals/inverts.
You'll also probably have a small amount of bacteria die-off in your LR, but IME it's usually pretty negligible.
If this doesn't sound like something you want to do, then your only other option is to remove
all of the fish in your DT, let it sit fishless for at least 9 weeks and treat the fish in separate QT(s). The 9 weeks is so that the parasites will starve out without a fish host to feed on. You can throw a pinch of flake in the DT every few days to maintain your bacteria levels.