^^ I basically agree with this, although I think you could put the fish in a bucket (temporarily), do a 80-90% WC on the QT and then resume with the copper. Get it back up to the same level it is now (0.25 ppm) before adding the fish back to the QT. I base this answer upon people I've helped in the past who did the same thing (added an ammonia reducer to a QT with copper), and this strategy seems to have always worked. You can worry about getting it up to 0.5 sometime tomorrow. Not a perfect answer I know considering the situation (velvet), but it's the lesser of two evils at this point.