You don't need a freshwater tank for dips. Get a big glass bowl, or a Tupperware container, get your fresh water (RODI
or at least RO
) the same ph and temp as your DT, get the fish in there, watch very carefully for distress. I've read some fish can go 30 minutes, some people suggest 15-30 minutes. Most suggest in the 5 minute ballpark. I go the shorter time. I've done it a few times and never had a problem. Not sure I'd be comfortable going more than ten minutes. If you watch the fish and pull it if it starts to look bad, it should be okay.
Copper treatments scare me. Copper can easily kill a fish if overdosed. Hypo (for stuff like Ich) tends to be less threatening, at least in my mind, because if you happen to be a little high, the worst case scenario is that its not killing all the parasites as quick.