How to do a 100% water change for a sick fish? My betta is sick with velvet....

[cheeryisgood]

...Easily stressed as well. Pls help I always use siphoning method. But since my betta has velvet and is lethargic and is clamping his fins, I had no choice but to move him into a quarantine container. He was VERY stressed out when I had to change the entire water to new water despite being aged water and treated with Tetra Aqua safe.

I asked around the betta forum and they say I'd have to do a 100% water change daily + aquarium salt + medicine.

But I don't know how to change water without stressing the fish. >_< How long does it take for water to settle to room temperature? I've even kept water in my room for a night to get room temp and my fish is still stressed.

If I use siphoning method and leave my fish in the quarantine container (taking him out to a new container stresses him even more), I can probably only change 80% of the water. 20% of the old water from yesterday remains.

Please answer asap in detail. His life depends on this.
My fish doesn't live in a community tank and it doesn't have a heater or a filter. That's why I change water 20-35% every 2 days but it still has the white spot on his head which I've never seen before and shining spot at his throat. His is a black betta. Melano Black and I dun think they shine like that.

He was constantly playing or scratching himself on the moss since day 1 and I thought he was playing. Until it started to do it so many times every single day. I felt worried. Then the white shining spot starts appearing on him.

Since velvet is contagious, I will probably have to wash the original tank/disinfect it and throw away all the plants in side.