I would argue that copper will dissolve straight into the water and it certainly can react with substances in the water, such as nitrates to form copper nitrate.
QUOTE from water supply company: How does copper get into my drinking water?
The major sources of copper in drinking water are corrosion of household plumbing systems; and erosion of natural deposits. Copper enters the water (“leaches”) through contact with the plumbing. Copper leaches into water through corrosion – a dissolving or wearing away of metal caused by a chemical reaction between water and your plumbing. Copper can leach into water primarily from pipes, but fixtures and faucets (brass), and fittings can also be a source. The amount of copper in your water also depends on the types and amounts of minerals in the water, how long the water stays in the pipes, the amount of wear in the pipes, the water’s acidity and its temperature.
Anyway, if you are changing less than 20 litres the easiest way is to get a spare heater and place that in the fermentation bucket a couple of hrs before you need the water. A power head won't do any harm either. This is the method I used before setting up my latest tank which is 100gallons, so I put the water straight into the tank now as I change 40 gallons at a time.