You can put an aerator wherever there is threading for one.--I'm not sure of your description "brass faucet valve". - The end of the spout is generally threaded for an aerator.
Female x Female coupling
At the very tip of the spout.
A clogged aerator on the end of the faucet would cause the water flow to decrease in that faucet.
Normally at the end of the faucet spout
Unscrew the aerator and clean it and reinstall it
Probably a clogged aerator. Remove the aerator and clean out the screen and reinstall.
Clean the aerator
Mineral deposits; most are formed in your pipes when water is not flowing. The aerator can be removed and cleaned extending the life of the aerator. The aerator will eventually deteriorate and should be replaced. Depending on your water supply and the pipes, the minerals can be several different colors.
well if you have a plugged aerator or your water line is corroded then that is usually the reason a faucet has low pressure if its just one faucet.
You need a special adaptor to link the two. This is an insert, threaded at one end to fit where the faucet aerator fits (you remove the aerator) and at the other end a pipe thread to fit a shower hose. These are commonly available in most plumbing stores and there are 2-3 variations. -Take your aerator along for comparison when buying the adaptor.
An aerator is on the end of your kitchen or bathroom faucet spout that unscrews and holds the screen and/or water saver.