Freshwater dips does help for Velvet. You only dip them max up to five minutes. 10-15mins is way to long. Freshwater dips help relieve the velvet but if your sticking the fish right back in the infected tank they will just get infected again. Only thing is copper away in a cycled Qt tank.
If tank is not cycled yet. You can use the transfer method to tie you over. Using rubber maid containers, add heater, used filter media from main tank, an aquaclear and powerhead if got it. Then freshwater dip all fish, one by one (matching temp & Ph) & then add fish after done to the new rubbermaid container tub (premixed) of same salinity, temp, & PH as main tank. You would have to do this every three days, the transfer method as this parasite can complete its life cycle in as little as three-five days. This transfer method is good to use until QT is ready to go. Its a pain but helps and also already starts your main tank following away its best to leave tank fallow for the 6weeks at least.
Great link to read on Marine Velvet & freshwater dips and its effects with Velvet.
http://www.petsforum.com/personal/tr...loodinium.html