Tricky thing, the water changes, hm? I use a piece of airline tubing, and very carefully go over the floor of the tank. And the water doesn't run into the drain, I let it run into a bucket, and if I accidentally sucked up any goldies, I see them swimming in the bucket. A scoop up with a plastic container, and the escapees are back in the tank.
New water goes back into the tank the same way - a bucket of temperature matched and de-chlorinated water, sitting above the tank someplace (mine sits on the tank light), and via airline tubing the water slowly rins back into the tank.
The water change takes a while at the beginning, simply because the fry are so darn small. The bigger the get, the easier it is gonna be.
As for how much - at first I only replace what I scooped off of the tank floor, and I don't go over the 6 inch mark in the tank either.
Today I did water changes on my 1 month old fry, and they are up to 8 inches now, and finally look like some mini fish, and not like some swimming stick with a pair of eyes!