Of course I mixed in tank water before I put it in D: Lol I put it in a cup of tnak water, swished it around for a couple mins, the poured it into a corner by the filter to mix it around more Then he went right over to where I dumped it and sat there while it was mixing/dissolving into the rest of the water.
Velvet takes ages and ages to be eradicated fully. Even raising the temperature and treating with medication. I had velvet go through all my tanks and it took at least three or so weeks before my fish recovered and there was no sign of it anywhere.
I agree that raising the temperature, completely blacking out the tank and using a medication is the best way to go. I found that a medication containing copper was what eventually knocked my velvet on its head. I know copper can be toxic etc. but in my case I had fatalities occurring and I did use the same medication on my fry tanks with no ill effects.
Also if your medicine contains malachite green I believe this is sensitive to light so it is best to keep tanks that are being treated with it dark anyway.
If I move the snail, I don't want it to reproduce, or contaminate the other tank... I'm already nervous about the heater I swapped out. I rinsed it under burning water and dried it real good, but I'm still worried.
If you use any copper medication on the tank, just know that the chances of it being invert-safe again are debatable. This is especially true if it is a glass tank with silicone, because the copper will soak into the silicone.