All my fish are now Dead!

except for my two blue chromis'. They look fine. It had to have been something way worse than Ick. Velvet maybe? I don't know. I did Quaranteen my fish before I put them in for 4 weeks, no signs so they went in. 1 month later, sick, 3 days later, DEAD! Some white spots, some white dusty, (velvety) covering, my Harlequin Tusk did do some rubbing on some rocks, and DEAD! I treated with Garlic and two different kinds of reef safe medications for parasites.

So, now I need to get my Chromis' out and put them in the Q tank for 8 weeks, in hyposalinity. The 8 weeks w/o fish for my show tank will allow ALL parasites to die off? Is this true or not? Do I need to do anything else. I think I'm going for a 2 month quaranteen for all new fish just to be sure.:sad

BTW here are my tank specs

55 gallon, wet/dry, skimmer, 60lbs LR.
When the first fish got sick I immediately tested the water and did an emergency water change. The test came back 0 on ammonia and Nitrite and ~15 on nitrate. I thought, oh well, maybe nitrates were bothering the fish so that's the reason for the water change. My SG was 1.026 and temp was holding steady at around 78 F.

I introduced this parasite by buying a fish from an LFS. He died and took some with him. It was one of my first fish, so I learned from my mistake and got a Quaranteen setup. (I know a little too late obviously) I waited a little over a month with no fish in the tank. after 4-5 weeks, I put my clown, Chromis' and lawnmower blennie back in the main tank. I waited another 2 weeks, and all was good. So I bought myself a new Tusk and a small exquisite wrasse, and quaranteend them for 4 more weeks, and all was well. They went in the tank and like I said, a few weeks later the wrasse got powdery and the Tusk got some spots, or dusty. 3 days later Tusk died, next day, Wrasse and baby clown die, and the day after that all were dead except the two chromis'

All I can figure is that I didn't give the parasite enough time to finish it's life cycle before I put my fish back in. (4 weeks) and so they were able to make a comeback but not sicken my fish. But the Tusk and Wrasse seemed very succeptable to it. I still dont' think it was Ick because I've read on here about people fighting off Ick for weeks and this **** killed in a matter or 72 hours.