Isn't Ich itself a parasite? How can something safe for inverts kill Ich?
Here's what I would do...
Use your QT. Set the temp in the low to mid 80s. Get ALL, yes not just those infected, but ALL fish out of the tank and put them into the QT. On their way, do a freshwater bath for about 3 minutes in water with the same pH as the tank.
Over a period of 48 hours lower the salinity of the water to 1.009-1.010. In addition, get some Cu from your LFS as well as a testing kit and adjust the Cu concentration in the water to .15 -.20. Keep the water at a gravity of 1.009-1.010 and a copper concentration of .15 -.20 for a period of 14 days. I might even keep the gravity in hypo for 30 days. After 14 days, do a water change to remove the majority of the copper.
KEEP THE FISH IN THE QT for a total of 30 days. While in the QT feed them food that was soaked in Garlic Guard. The ich will probably be gone, but these extra days will help the fish get stronger and healthier, but more importantly, it allows the Ich to runs it's lifecycle in your main tank. Just because you've removed the fish doesn't mean the Ich is gone. The Ich parasite, after leaving the host fish, hatches into 200 more and goes looking for a new host. When there are no hosts (because you've removed ALL your fish) the Ich will die after a period of 30 days.
If you only treat one of your fish, others will get Ich from the parasites in the water. If you put them back in too soon, the Ich will not be dead from the tank and you will reinfect them. YOU HAVE TO HAVE PATIENCE and ALWAYS use a QT in the future. The best cure is prevention!
People refrain from QTs because they dont want the $100 fish they just bought to sit in a separate tank for 30 days, but do you want to infect your whole system? 30 days is well worth keeping a healthy display tank.
Hope this helps and sorry for the lengthy post.