I had issues when I first started with marine velvet, and crypto.All were LFS fish I did not QT. After 7 weeks of a fallow tank, dosing Brightwell Aquatics Microbacter and feeding the tank a small amount of mysis once a week and doing water changes once a week I bought 6 small fish from a BRS member and have had no issues to date in my 40 gallon tank.
I then started a 150 gallon tank and bought some small fish including a small yellow tang from 2 different LFS. I QT'd for 3 weeks while the tank cycled but once I added them to the tank Crypto showed its ugly face. I hastily yanked the fish out and put them in my QT and started dosing cupramine. The two clowns survived but I lost the yellow tang- my bet being too much Cupramine. After another 7 week fallow period, dosing bacteria, and water changes I added the clownfish to the 150 and all was well. I saw another BRS member breaking down their tang dominant tank and bought them after watching them look super healthy in his mature tank. I took them and put them all in at once that night. All was well for 2 weeks and then crypto showed up again. I decided this time I would do massive 50% or more water changes every few days, feed a couple sheets of Nori each day, feed LRS fish frenzy soaked in Selcon, and also used polyp lab Medic at heavy dose. I lost my purple tang and 2 clownfish but everybody else survived and no signs of ich for several months.
I will QT all fish from now on but feel you can't eradicate crypto completely. I personally think it lives within the fish and is relatively dormant until the fish is stressed and its immune system comes down. Alot will disagree with me in that they feel ich can be eradicated but this is my observation in the short time I have been doing this. I wish you good luck!