I am not a marine disease expert, but I don’t think a freshwater dip will kill velvet or ich. To my knowledge, the only way to ensure the macro is velvet/ich free is to run it fallow (as I don’t know how macro would handle copper treatment, or if it would be invert safe after a copper treatment). 45 days at 81F in a fishless system should be sufficient to make sure the macro isn’t spreading ich/velvet (many people suggest 76 days to be safe on the disease front).
That said, if the disease is in your tank, then you’ll need to run the entire system (tank, sump, etc. - anything in contact with your tank’s water) fallow (fishless) for the 45 days at 81F, otherwise any surviving parasites will simply attach to any new fish you add and wipe put your tank again. Once the 45 day fallow period is over, your system will be safe for fish again, but you’ll want to quarantine everything you put in your tank (macro, corals, inverts, etc.) for 76 days and quarantine and treat any new fish (Jay Hemdal describes how to do this properly in his threads in the disease forum here on Reef2Reef) before putting them in your tank, or the disease could be introduced to your tank again.
