quarantine

[outtafocus]

I dont understand the use of the daily 20% water change.
What purpose does this serve?

I currently have a bicolor blenny in my QT, I have not medicated this tank. I use it mostly to observe the fish. A black clown I picked up last week just got moved into my 25gal as he was showing no ill signs and is doing well in the 25.
The last time I QT'd a pair of clowns I used Seachems Cupramine with horrible results, I lost both fish. I believe this was due to the panic use of the QT. I had added the fish to my 10gal and within a day they developed white dots, knee jerk reaction was to dump both into a QT and add cupramine. I believe there was an ammonia spike and both fish died within 2days. I feel that if I had left the fish as they where in the 10gal, they would still be alive.
Several people have told me they do not use a QT.